I see what you're saying. Technically it could just parse the instance from a string like that or at least let us type the name and instance in the fields manually instead of adding it.
Honestly, the liftoff app has a great search feature. Have you tried adding the instance where that community is, searching for the community in that instance and then tap subscribe and it'll transfer you to your instance view of that community.
For some reason I find stock GNOME UIs appealing