You get into the Lobby after you have logged in.
Challenging
Search for a player in the player-list in the bottom right section.
Highlight the player's name with your mouse. The "Challenge" button at the very bottom becomes active.

Press the "Challenge" button, and wait for your opponent to accept or reject your challenge.

You can cancel the challenge any time if your opponent does not repond within a reasonable time.
Alternatively you can wait for other players to challenge you.

You can ACCEPT or REJECT the incoming challenge.
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