I believe this can best be done using the built-in app WebCoRE. I've included screenshots of a mock Piston wherein these two things happen:
(1) Define a variable, called Payload here, containing the XML you pasted above (instead of having it saved into a file);
(2) Execute a Web request, in the form of a PUT, sending the aforementioned variable to the special URL (which also could have been passed via variable, but for the sake of illustration, I just hard-coded it into the PUT task, as shown).
The problem with using Rule Machine -- the traditional powerhouse app for automations in HE -- for this is that RM does not permit certain special/HTML characters in variables, even if you escape them. The second thing to realize is that, even though your Hubitat has a File Manager (like Vera), there's no immediate facility I'm aware of for sending out the contents of a file. Someone will surely correct me if I've missed something, such as whether special considerations need to be taken for sending pre-formatted TEXT instead of JSON. I'm not a WebCoRE expert.
P.S. WELCOME TO HUBITAT, because it runs rings around your old hub!!
