I remember to have the same issue a couple of days ago - images continued to be sent to nova.astrometry.net instead to ansvr, even if I had the domain field point to my ansvr setup.
My ansvr is running on Port 8080. I managed to get APPM working with my local ansvr by putting 127.0.0.1:8080 into the domain field - make sure to include the port there. The ping button will not work then, likely as the ping command does not allow to ping an IP on a specific port and will fail if the domain string is used as a parameter - i.e. the domain field will be marked in red, indicating a wrong setup, despite that the domain is setup correctly in this case. (If you leave out the port, the ping to localost will obviously be successful, and the domain field green - but ansvr will not receive images! So I feel the ping test button can be misleading here, despite it was put in with good intention.)
I think when I left the API key field blank, APPM crashed with a runtime exception, so I entered the key from my nova.astrometry.net account there (to be sure the string has the right format and length). I tested it, and my images do not get sent anymore to nova.astrometry.net, but to ansvr now. You can see if it works by watching the ansvr output in the command line window (launch "watch ansvr log").
Hope that helps.