Xojo Plugin Requires Chilkat v11.7.0+
Box: Create a Shared Link with Explicit Access

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. Create a collaborators-only link for a file. It points existing collaborators to the file; it does not invite new users. Wider access levels are subject to enterprise policy.
See the Box API guide for setup and the complete workflow.
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Xojo Plugin
Dim success As Boolean
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.
Dim curl As New 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"
Dim setupCommand As New Chilkat.StringBuilder
Dim setupReplacements As Int32

//  Register findParent; registration sends no request.
success = 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 <> True) Then
    System.DebugLog(curl.LastErrorText)
    Return
End If

//  Register findFolder; registration sends no request.
success = 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 <> True) Then
    System.DebugLog(curl.LastErrorText)
    Return
End If

//  Register findFile; registration sends no request.
success = 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 <> True) Then
    System.DebugLog(curl.LastErrorText)
    Return
End If

//  Register findDestination; registration sends no request.
success = 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 <> True) Then
    System.DebugLog(curl.LastErrorText)
    Return
End If

//  Register getFileDetails; registration sends no request.
success = 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 <> True) Then
    System.DebugLog(curl.LastErrorText)
    Return
End If

//  Register getEnterprise; registration sends no request.
success = 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 <> True) Then
    System.DebugLog(curl.LastErrorText)
    Return
End If

//  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 <> True) Then
    System.DebugLog(curl.LastErrorText)
    Return
End If

success = curl.AddOutput2("findFolder","entries","name","{{folder_name}}",True,"id","folder_id")
If (success <> True) Then
    System.DebugLog(curl.LastErrorText)
    Return
End If

success = curl.AddOutput2("findFile","entries","name","{{file_name}}",True,"id","file_id")
If (success <> True) Then
    System.DebugLog(curl.LastErrorText)
    Return
End If

success = curl.AddOutput2("findDestination","entries","name","{{destination_name}}",True,"id","destination_id")
If (success <> True) Then
    System.DebugLog(curl.LastErrorText)
    Return
End If

success = curl.AddOutput("getFileDetails","file_version.id","file_version_id")
If (success <> True) Then
    System.DebugLog(curl.LastErrorText)
    Return
End If

success = curl.AddOutput("getFileDetails","sha1","file_sha1")
If (success <> True) Then
    System.DebugLog(curl.LastErrorText)
    Return
End If

success = curl.AddOutput("getEnterprise","enterprise.id","enterprise_id")
If (success <> True) Then
    System.DebugLog(curl.LastErrorText)
    Return
End If

//  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.
Dim parentName As String
parentName = "/"
Dim folderName As String
folderName = "images"
Dim fileName As String
fileName = "penguins.jpg"
If ((curl.GetVar("parent_name") <> parentName) Or ((parentName = "/") And (curl.VarDefined("parent_id") = False))) Then
    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 = "/") Then
        curl.SetVar "parent_id","0"
    End If

End If

If (curl.GetVar("folder_name") <> folderName) Then
    curl.SetVar "folder_name",folderName
    curl.ClearVar "folder_id"
    curl.ClearVar "file_id"
    curl.ClearVar "file_version_id"
    curl.ClearVar "file_sha1"
End If

If (curl.GetVar("file_name") <> fileName) Then
    curl.SetVar "file_name",fileName
    curl.ClearVar "file_id"
    curl.ClearVar "file_version_id"
    curl.ClearVar "file_sha1"
End If

curl.ClearTargetOutput "*"
curl.ClearVar "shared_url"
curl.AddTargetOutput "shared_link.url","shared_url"
Dim authorizedCommand1 As New Chilkat.StringBuilder
success = authorizedCommand1.SetString("curl -X PUT ""https://api.box.com/2.0/files/{{file_id}}?fields=id,shared_link"" -H ""Authorization: Bearer {{access_token}}"" -H ""Content-Type: application/json"" -d '{""shared_link"":{""access"":""collaborators""}}'")
Dim headerReplacement1 As Int32
headerReplacement1 = authorizedCommand1.Replace("{{access_token}}",curl.GetVar("access_token"))
success = curl.DoYourThing(authorizedCommand1.GetAsString())
If ((success <> True) Or (curl.StatusCode <> 200)) Then
    System.DebugLog("Box request failed. HTTP status: " + Str(curl.StatusCode))
    System.DebugLog("Failure reason: " + Str(curl.FailReason))
    System.DebugLog(curl.ResponseBodyStr)
    Return
End If

If (curl.VarDefined("!") <> True) Then
    System.DebugLog("Expected response fields are missing.")
    Return
End If

System.DebugLog(curl.GetVar("shared_url"))