Activating an Avengers Square Enix key is really simple, just follow these steps:
1.Go to Avengers Redemption page on the Square Enix page here
2.Login or register a new Square Enix Members Account
3.Put the purchased “18 character code” into the website and choose the desired platform (PC/PS4/XBOne)
Keep in mind that you must be signed into Marvel’s Avengers with your Square Enix Members credentials in order to access rewards and bonus content in-game.