When attempting to set up a Web Service step in a workflow that utilizes a GET request containing a body, the following error message is shown;

"415 UnsupportedMediaType" 

By product design, the Web Service activity does not support a GET request with a body. The body will be ignored when sending the request.
To achieve this desired outcome, it is necessary to define a POST method with the same logic as the GET and change the method signature of the web service activity to POST as well. 

KBA is applicable for both Cloud and On-premise Organizations.