Hey dmeilhac
you have the postquerystring checkbox ticked - this means that if your URI is '/resource-name?param1=value1¶m2=value2' (so you have two QUERY type parms setup on the request, instead of them appearing in your request's URI, you can move them to the payload field).
I don't think your XML payload is supposed to map to a QUERY parameter on your request. Either something else is (a different parameter value) or none at all Im guessing. Youve blanked out the request's host and URI in the screenshot so I cant see whats going on. For security considerations, all you need to do is blank out the host.
So if you have a request with full URL of
https://myserver.com/resource-name?param1=value1¶m2=value2
the only bit you need to blank out is the host bit '//myserver.com'
Can you show the URI (the bit AFTER the host) so we can see what your QUERY parms look like? Then we'd be able to see what's going on. Obviously I don't know what PARMS you do have in your URI, but looking at the scrubbed/blanked bit - it doesnt actually look like you have any QUERY parms, so at this point, without knowing anything else Id remove the QUERY parm completely (or set it to PLAIN so it isn't used).
Have a look at this link as this explains things quite well
cheers,
rich