Forza Horizon 3 XBOX One / Windows 10 CD Key

Go to https://www.xbox.com


Login to your account


Hover over your account link (the top right of the screen) a menu will appear, click on the "Redeem Code" button.


On the next page, click "Redeem"


Enter your code


Click "Confirm"


Click "Confirm" again to assign the key to your account.