Found sort of a solution - use RegisteredHandler's create event to subscribe to the room's event.
The changes can be found here.

I wanna make an important note here:

Creating an async main method in index.ts and awaiting gameServer.register was crucial.

I have a flow that when a client enters the game, I use getAllAvailableRooms to get all game lobbies a client can join, or allow him to create a new one (See the PR I sent to colyseus).

After they join a game lobby, they basically join a GameLobby room, which waits until all clients are ready and the owner presses Start and the game should then start.

My idea is that I should be able to create a room from inside the onMessage function,
which would in turn give me the newly created roomId, which I can broadcast to my players with a start_game action.

But unfortunately, a room cannot access the matchmaker...

Given that - how would you go and make something like that?

Btw, this is the repo for code example and references.

As stated here, When trying to get AvailableRooms in the unity client, the metadata field does not deserialise correctly.