PowerShell Requires Chilkat v11.7.0+
Box: Upload a Local File with HttpCurl

Run the reusable setup once per HttpCurl object, then select names and execute the operation. The setup is repeated here only to make this published example independently runnable. Upload a local PDF into documents, selected by name. Resolve the folder with the registry before building multipart attributes. This explicit second phase avoids literal placeholders in -F fields; files are limited to 50 MB for direct uploads.
See the Box API guide for setup and the complete workflow.
Chilkat PowerShell Downloads

Download Chilkat for PowerShell
PowerShell
Add-Type -Path "C:\chilkat\ChilkatDotNet47-x64\ChilkatDotNet47.dll"

$success = $false

#  Requires Chilkat 11.7.0 or later and a previously unlocked Chilkat bundle.
#  See Global Unlock Sample.
#  Guide: Box API Guide.

$success = $false

#  BEGIN ONE-TIME SETUP
#  This block is repeated so each published example can run on its own.
#  In an application, run the setup example once and pass the SAME curl object
#  to operation code below. Do not recreate it for every operation.
#  See Reusable Box HttpCurl Setup.
$curl = New-Object Chilkat.HttpCurl
$curl.SetVar("access_token","BOX_ACCESS_TOKEN")
$curl.SetVar("parent_name","/")
#  Box's fixed root ID is an implementation detail; callers supply names.
$curl.SetVar("parent_id","0")
$setupCommand = New-Object Chilkat.StringBuilder

#  Register findParent; registration sends no request.
$setupCommand.SetString("curl -X GET `"https://api.box.com/2.0/folders/0/items?limit=1000&fields=id,type,name`" -H `"Authorization: Bearer ACCESS_TOKEN`"")
$setupReplacements = $setupCommand.Replace("ACCESS_TOKEN",$curl.GetVar("access_token"))
$success = $curl.AddFunction("findParent",$setupCommand.GetAsString())
if ($success -ne $true) {
    $($curl.LastErrorText)
    exit
}

#  Register findFolder; registration sends no request.
$setupCommand.SetString("curl -X GET `"https://api.box.com/2.0/folders/{{parent_id}}/items?limit=1000&fields=id,type,name`" -H `"Authorization: Bearer ACCESS_TOKEN`"")
$setupReplacements = $setupCommand.Replace("ACCESS_TOKEN",$curl.GetVar("access_token"))
$success = $curl.AddFunction("findFolder",$setupCommand.GetAsString())
if ($success -ne $true) {
    $($curl.LastErrorText)
    exit
}

#  Register findFile; registration sends no request.
$setupCommand.SetString("curl -X GET `"https://api.box.com/2.0/folders/{{folder_id}}/items?limit=1000&fields=id,type,name`" -H `"Authorization: Bearer ACCESS_TOKEN`"")
$setupReplacements = $setupCommand.Replace("ACCESS_TOKEN",$curl.GetVar("access_token"))
$success = $curl.AddFunction("findFile",$setupCommand.GetAsString())
if ($success -ne $true) {
    $($curl.LastErrorText)
    exit
}

#  Register findDestination; registration sends no request.
$setupCommand.SetString("curl -X GET `"https://api.box.com/2.0/folders/0/items?limit=1000&fields=id,type,name`" -H `"Authorization: Bearer ACCESS_TOKEN`"")
$setupReplacements = $setupCommand.Replace("ACCESS_TOKEN",$curl.GetVar("access_token"))
$success = $curl.AddFunction("findDestination",$setupCommand.GetAsString())
if ($success -ne $true) {
    $($curl.LastErrorText)
    exit
}

#  Register getFileDetails; registration sends no request.
$setupCommand.SetString("curl -X GET `"https://api.box.com/2.0/files/{{file_id}}?fields=id,sha1,file_version`" -H `"Authorization: Bearer ACCESS_TOKEN`"")
$setupReplacements = $setupCommand.Replace("ACCESS_TOKEN",$curl.GetVar("access_token"))
$success = $curl.AddFunction("getFileDetails",$setupCommand.GetAsString())
if ($success -ne $true) {
    $($curl.LastErrorText)
    exit
}

#  Register getEnterprise; registration sends no request.
$setupCommand.SetString("curl -X GET `"https://api.box.com/2.0/users/me?fields=enterprise`" -H `"Authorization: Bearer ACCESS_TOKEN`"")
$setupReplacements = $setupCommand.Replace("ACCESS_TOKEN",$curl.GetVar("access_token"))
$success = $curl.AddFunction("getEnterprise",$setupCommand.GetAsString())
if ($success -ne $true) {
    $($curl.LastErrorText)
    exit
}

#  Register output rules once. Chilkat 11.7.0 evaluates {{name}} variables
#  when extracting each output, using the current value at that time.
$success = $curl.AddOutput2("findParent","entries","name","{{parent_name}}",$true,"id","parent_id")
if ($success -ne $true) {
    $($curl.LastErrorText)
    exit
}

$success = $curl.AddOutput2("findFolder","entries","name","{{folder_name}}",$true,"id","folder_id")
if ($success -ne $true) {
    $($curl.LastErrorText)
    exit
}

$success = $curl.AddOutput2("findFile","entries","name","{{file_name}}",$true,"id","file_id")
if ($success -ne $true) {
    $($curl.LastErrorText)
    exit
}

$success = $curl.AddOutput2("findDestination","entries","name","{{destination_name}}",$true,"id","destination_id")
if ($success -ne $true) {
    $($curl.LastErrorText)
    exit
}

$success = $curl.AddOutput("getFileDetails","file_version.id","file_version_id")
if ($success -ne $true) {
    $($curl.LastErrorText)
    exit
}

$success = $curl.AddOutput("getFileDetails","sha1","file_sha1")
if ($success -ne $true) {
    $($curl.LastErrorText)
    exit
}

$success = $curl.AddOutput("getEnterprise","enterprise.id","enterprise_id")
if ($success -ne $true) {
    $($curl.LastErrorText)
    exit
}

#  END ONE-TIME SETUP
#  The name lookups below assume each listing fits in one page (1000 items).
#  Parent and destination names select root children; folder_name selects a child
#  of parent_name. "/" selects the root parent. These are not arbitrary paths.

#  BEGIN OPERATION -- keep the configured object when reusing it.
#  Inputs are names. Retain cached IDs when the selected names have not changed.
$parentName = "/"
$folderName = "documents"
if (($curl.GetVar("parent_name") -ne $parentName) -or (($parentName -eq "/") -and ($curl.VarDefined("parent_id") -eq $false))) {
    $curl.SetVar("parent_name",$parentName)
    $curl.ClearVar("parent_id")
    $curl.ClearVar("folder_id")
    $curl.ClearVar("file_id")
    $curl.ClearVar("file_version_id")
    $curl.ClearVar("file_sha1")
    if ($parentName -eq "/") {
        $curl.SetVar("parent_id","0")
    }

}

if ($curl.GetVar("folder_name") -ne $folderName) {
    $curl.SetVar("folder_name",$folderName)
    $curl.ClearVar("folder_id")
    $curl.ClearVar("file_id")
    $curl.ClearVar("file_version_id")
    $curl.ClearVar("file_sha1")
}

#  Resolve the named destination first. HttpCurl does not expand variables
#  inside multipart -F attributes in the tested build, so this upload is two phases.
$curl.ClearTargetOutput("*")
$authorizedCommand1 = New-Object Chilkat.StringBuilder
$authorizedCommand1.SetString("curl -X GET `"https://api.box.com/2.0/folders/{{folder_id}}?fields=id`" -H `"Authorization: Bearer {{access_token}}`"")
$headerReplacement1 = $authorizedCommand1.Replace("{{access_token}}",$curl.GetVar("access_token"))
$success = $curl.DoYourThing($authorizedCommand1.GetAsString())
if (($success -ne $true) -or ($curl.StatusCode -ne 200)) {
    $("Box request failed. HTTP status: " + $curl.StatusCode)
    $("Failure reason: " + $curl.FailReason)
    $($curl.ResponseBodyStr)
    exit
}

#  The file is read from the path following @. Keep attributes before file.
#  Resolve the multipart attributes field before passing the command to HttpCurl.
#  Box folder IDs come from configuration or a previous API response.
$uploadCommand = New-Object Chilkat.StringBuilder
$authorizedCommand2 = New-Object Chilkat.StringBuilder
$authorizedCommand2.SetString("curl -X POST `"https://upload.box.com/api/2.0/files/content`" -H `"Authorization: Bearer {{access_token}}`" -F 'attributes={`"name`":`"invoice.pdf`",`"parent`":{`"id`":`"{{folder_id}}`"}}' -F `"file=@qa_data/pdf/invoice.pdf;type=application/pdf`"")
$headerReplacement2 = $authorizedCommand2.Replace("{{access_token}}",$curl.GetVar("access_token"))
$uploadCommand.SetString($authorizedCommand2.GetAsString())
$replacements = $uploadCommand.Replace("{{folder_id}}",$curl.GetVar("folder_id"))
$curl.ClearTargetOutput("*")
$curl.ClearVar("file_id")
$curl.AddTargetOutput("entries[0].id","file_id")
$success = $curl.DoYourThing($uploadCommand.GetAsString())
if (($success -ne $true) -or ($curl.StatusCode -ne 201)) {
    $("Box request failed. HTTP status: " + $curl.StatusCode)
    $("Failure reason: " + $curl.FailReason)
    $($curl.ResponseBodyStr)
    exit
}

if ($curl.VarDefined("!") -ne $true) {
    $("Expected response fields are missing.")
    exit
}

$("Uploaded file ID: " + $curl.GetVar("file_id"))
$curl.SetVar("file_name","invoice.pdf")
$curl.ClearVar("file_version_id")
$curl.ClearVar("file_sha1")