In order to use the key follow the instruction below:
1. Go to https://www.xbox.com2. Login to your account3. Hover over your account link (the top right of the screen) a menu will appear, click on the "Redeem Code" button.4. On the next page, click "Redeem"5. Enter your code6. Click "Confirm"7. Click "Confirm" again to assaign the key to your account.