You are getting back a 415 error which usually means you’ve not set a Content-type HTTP header, but it could also be related to Content-Encoding header.
Have a look at the documentation for your API, and see if it says that a Content-Type HTTP header has to be sent.
If it does ask for this, in your Call REST action, go into the HTTP Headers tab, and add one as a Custom HTTP Header. You’ve probably want something like “Content-type” for the key and “application/xml” or “text/xml” for the value, but do check the documentation.
Hope this helps.
we do not know how the HTTP header should look like. Is it part of the XML? Our XML looks this way: