You can access the surrender feature by pressing the "ESC (Menu)" and then "Surrender" button.
Surrender feature can be used after 7 minutes into the match.

NOTE: Surrender feature is currently only available for Competitive Sabotage

After using the Surrender Button you will have 40 seconds to vote.
You can vote with the "F9" key to confirm and the "F10" key to reject surrender.

If 4 players out of 5 confirm as a result of the voting, the match is over and you lose.
Otherwise, if 2 players out of 5 give vote to reject, surrender will be canceled and the match will continue from where it left off.
Note that these numbers vary according to the number of players in the team.

If the surrender is cancelled, you will not be able to start voting again until the sides change, except in certain exceptional circumstances.
For example, when a player from your team leaves, you get 1 extra surrender right in the same half.
In addition, both teams gain 1 more chance to surrender in overtime.
Only players who are alive can start the voting. If the vote has already been started you can still vote, even if you're dead (spectating).