Adding custom HTTP Headers is straight-forward; the Headers inspector at the bottom of the XML editor allows for this:
Here we’ve add a custom Content-Type header which will override the standard Content-Type used for the SOAP Request (“text/xml; charset=utf-8”). Sending the request and looking the Raw Request Viewer reveals
You can of course add as many desired headers as required, and their value can contain property expansions as usual.
The corresponding Headers tab for the response message not surprisingly shows all HTTP Headers in the response: