If you don't need send extra information in other post parameters the xml parameter name is redundant and innecesary as your teammate said, if the 3rd party waits only for a XML data only send the raw data in the POST body with the correct mime type and encoding and and do not complicate.
The process for Getting raw data is easy in most application server containers, so you dont care about that, most of them uses a Reader to get received data and manipulate it.