API - Posting widget updates via powershell

The multipart boundary stuff is more related to sending large amount of data like XML file contents while most API calls including widget updates just need to pass direct JSON content. You might want to look at the examples at https://www.logicmonitor.com/support/rest-api-developers-guide/v1/rest-api-v1-examples/#Example-PowerShell-GET which uses JSON content.

I also think that JSON always starts and ends with "{" or "}" so you might need to add that to your $data string. Also since you are using PowerShell, you can use ConvertTo-JSON to convert PSObjects to JSON if that makes it easier to build and modify.