Home.

Somebody needs to start the lobby.

Launch L4D2.

Play Campaign > Play With Friends > Create new Lobby

Change Official Dedicated to Best Available Dedicated.

Enter mm_dedicated_force_servers "otat.xyz:27018|10.10.2.10:27018,192.168.10.10:27018" in the console.

Then start lobby as usual.

Then once the lobby is open, others join with the connect link.