how do I make a leaderboard with an online multiplayer game?
The same way you do with single player, set up some event that will trigger the leaderboard and send the current player name's score to the leaderboard. You can find the expression for the current player's username from the expression builder list.
If you want only one score sent for the team of players, you could use the condition to check if this player is the host, and then only send the leaderboard info from the host's game.