how to begin games in waiting room

How can I accept a challenge from someone in the Waiting Room ?

Answer: you can't. The 'waiting room' is just that - for waiting. Eventually, the computer will start your game against another player also waiting. if players added conditions, such as limiting the rating range, number of players, etc., then those conditions must be met.
The ONLY way for players to start games on their own is by challenging other players directly and having the challenge accepted. These games are not rated.
meow.

It would be really nice if you were able to open seeks for games like on Lichess or Board Game Arena. I've been sending invites to some regulars on Dameo, but only because I can't extend open invitations.

PS challenge me to Dameo if you're interested