H477YH0UR2017 NO LONGER USABLE.
It contains a mission for 5x experience for a victory, doable for 5 times. You need to complete the 5x bonus for first victory, though, so it’s not stack-able. Missions activates automatically after redeeming the code.
Could be a really helpful tool if you have anything to grind, since you can repeat the missions with the same tank.