Sorry, but it’s not possible at the moment.
Actually, it’s possible.
You will know that an onion domain supports subdomains.
It’s not about subdomaining. There is no way to make the forums work well with the Tor browser at highest security settings due to all the JS.
I still connect to forums.localmonero.co on tor browser…
With low/medium safety settings, I presume?
I use standard safety settings.
Connecting to forums.localmonero.co on tor browser beats connecting to it on clearnet.