If it's your turn, you can just click the orange "Mah Jongg" button above your tiles.
If you want to call a discard for Mah Jongg, you can either:
- Click "Call", select "Mah Jongg", click "OK".
- Or just click the orange "Mah Jongg" button above your tiles
- Or press M on your keyboard
Caution: if this is not a valid Mah Jongg, your hand will be called dead and you will not be allowed to continue playing in this game.