How to initiate multiplayer without command line?

You can call ClientTravel() in your PlayerController class to have a client join a game without the user having to do anything via the command line. If your server will have a static IP, that’s all there is to it. If not though, the bigger issue is finding servers for the clients to join. Check out the ShooterGameSession class in ShooterGame for an example of finding matches/matchmaking.