The issue says it isn’t accessible through the GUI. The Asterisk documentation for it is:asterisk/pjsip.conf.sample at master · asterisk/asterisk · GitHub
The general way of customising the configuration is given in Best way to add missing GUI configuration options to PJSIP endpoint - #2 by billsimon although I think a slightly different file is needed for an AOR, and I would suggest adding type=aor, after the +.
You might also want to set the max contacts to 1, which I think can be done from the GUI, and just might default to remove existing.