It would be best not to start a multiplayer game if you might have to quit it.
Second best is to use the "quit" or "concede" tab in the game's options menu, after an excuse in the chat.
What do you mean by "without counting"?
It will count against completion %% for you and all affected players,
there will be no score - futile protection of the average,
it may or may not count against you in match stats.