Sorry, my French isn't good enough to type the instructions in it...
Under the gear in the upper right corner, look for the handshake labeled "Proposer un abandon collectif de la partie"
Once the proposal has been made, players can vote if they want to abandon the game or not.
Important:
All players must vote yes for the game to be abandoned. Even one player voting no or not voting will keep the game going.
Also important: Proposing to abandon does not stop your clock, or prevent time-out (or leave) penalties.
If all players do agree to abandon the game, the system treats it as if the game never happened: there is no ELO adjustment, number of games played does not change, you can't give public thumbs to the players (as you haven't played a game with them recently...) et cetera.
And, some too long, didn't read over-explanation....
This is quite different from the conceding a game (white flag) option found in the same drop-down menu. Abandoning is available no matter how many players are available; conceding only when there are two players. Abandoning requires everyone to vote yes; conceding only requires one person. Abandoning thinks the game never happened; conceding ends the game with the non-conceding player winning (with full ELO and other statistics adjustments).