Can you share a copy of the request/response you’re using so that we can help debug? If you’re able to get the request to work in Postman, please double check that you’re passing all of the same headers and authentication parameters in your code.
Thank you for providing additional information. It looks like this is happening because you are using a Content-Type of application/xml when this endpoint only supports JSON. In my requests, I typically don’t supply a Content-Type header and based on the JSON body that the server gets, it’s able to respond as expected. If the tool/server you are using to make the request is setting this header, I would override it with the value application/json.