What @OgNasty said is correct in that we are not logging into this forum with signed message but rather with username and password. Signed message is only needed to recover your account if it gets hacked or lost, the recovery team needs it to prove ownership.

You also don't need to post your bitcoin address publicly (if it's private), but you may need to do that somewhere like a private PM or something. This can also help more than nothing. Also forum don't need your main address to sign messages, but just an empty address completely controlled by the user himself is more than enough.