Hey there!
Wix-Fetch can indeed send requests in any support HTTP format, including XML. You’d need to use the POST method to talk to your server, so you can include a body to your request. That body is the same information you might sending using cURL with the -d option, and should contain your XML string.
Here’s an example that you can use (The second one that says ‘post data to a resource’), where you can modify the Content-Type and body to match XML.
Quick question though, am I right in thinking you’re making a request to a SOAP API on that 3rd party server?