Hi Juan, it looks like it should work, is that a valid login for the QMC in normal circumstances? Have you also exported the certificate (you should get a warning in postman if the cert is invalid).
In postman, I have the following:
URL: https://<server URL>:4242/qrs/app/full?xrfkey=0123456789abcdef
UserDirectory=<my domain>;UserId=<my username> (my user is a rootadmin in QMC, but I don't think it should matter as long as the user can login and do the same task in QMC)