This article is for the redemption of single year access GameMaker licences (Creator, Indie, Enterprise). This is not relevant for Subscriptions.
If you have been given a GameMaker single year access code, you can redeem it to claim your copy of the product from your YoYo Account page (if you do not have a YoYo Account then you will first need to make one - more details here). First you should log into your YoYo Account and then go to the Redeem section of the Account Overview:
Once here you should enter the code that you have been given in the Code Entry box and then click the button marked Confirm.
This will then show you the Product that the Code will grant you. You will need to tick the checkbox accepting the terms and conditions to continue.
This will will add the single year access GameMaker licences to your account and it will be shown in the Subscription section of the Account Overview