Can't log in with Reddthat?

My instance isn't on the dropdown either, but you can just type it in that field.

The drop-down only shows a few instances, generally whichever ar emost popular. Just type or copy/paste. If you use a password manager, copy the URL into it and the you can just copy paste the URL any time you need to login instead of typing it (I've never had to retype my login on Jerboa but you never know).