This can be completed in either a public KOTH match or in a private one, or a combination of the two.
This requires you to be a spectator in the match, rather than one of the two fighters. Once the fighters have selected their map, a small menu will come on screen for only 5 seconds! Be prepared, or you will miss it. Usually, unless you are boosting, it's best to vote for the King (left).
Yesterday I got 8 out of my 10 correct votes in public matches in about half an hour. You can view player stats by pressing
. If a king has a streak going, it will probably keep going. There are a good number of skilled players who get into KOTH then maintain a streak for a long time, so, like I said, it's usually best to vote for the king.