Fists of Fury: Velen - Velen
Location: Crow's Perch
Quest Giver: Crow's Perch Notice Board
Other Requirements: You can also accept this quest by coming across one of the fights taking place
There are two ways for players to accept this quest, you can either find it on the Crow's Perch Notice Board (the same town you meet the Bloody Baron) or by stumbling upon any of the fights out in the world. One of the available fist fights is shown in my screen shot below. To initiate one of the fights speak with the Bookie who is taking bets on the fight.
All of these fights are extremely easy - after all you're a Witcher fighting regular humans. Fast travel to each location for the quest and pound in the face of the person the quest tells you to. Only one of the fighters, Fishgulper, asks you to throw the fight for them. Throwing the fight will not get you credit and as far as I know doesn't change the story at all.
If you do decide to throw the fight, you can talk to Fishgulper after the fight and offer him some Crowns anyway, as a kindness. Once you've defeated all three of the people the quest tells you to it'll send you to Crow's Perch to fight the Sergeant there.
Defeating the Sergeant completes the quest. If this is the final Fists of Fury quest that you're completing then you'll be approached by an NPC that gives you the final one, Fists of Fury: Champion of Champions.