This is kind of additional pain in framing an XML message and an alien way to send it as a SOAP request.
Because, ByD has already a structured way of processing soap messages. Check the studio documentation for Webservices and try out samples.
When you import wsdl's as External Webservice Integration modules, you will have reusable libraries created automatically based on wsdl structure which you can use to frame SOAP calls provided related communication arrangement is also setup.