To enable booster, at the moment, you need to be part of the testing group which we have picked, then you have the XP Booster available in the main hub, next to the Mission/Challenge button.
You can use the booster approx ten times per day, but this depends on your region and device.
You need to watch three advertisements to stack the XP Booster fully, and then you can play three matches with the booster.
You can as well play one match with the booster and after that instantly refill it, as you wish.
Leaving the match before it has ended, won't reward you any XP, so if you want to get the bonus XP from the XP Booster, you must play the game till it ends.