It is not possible to reach a v3 .onion page with only key, with 56 digits it misses exactly the .onion in the end.
Interesting, how common are v3 addresses I have not seen many of those? A possible workaround would be to put the TOR address into the username field with the .onion in the password field. You could also use options listed here - OnlyKey User's Guide | Docs
Yes, thanks, I am playing around with the advanced options now.
Selfhosting of services does make use of v3 a lot.