Please follow the steps below:
On PC
On a computer, go to “Redeem Code” and click “Sign In”.
Sign in with your Microsoft account.
Click “Redeem”.
Enter the 25-character prepaid code and click “Confirm”.
On Xbox One
Go to “Games”, and then select “Use a code”. Or, say “Xbox, use code”.
If prompted, sign in.
Enter the 25-character code.
Important Note: The 'stacking' of Xbox Live subscriptions is no longer allowed by Microsoft. Users will receive an error when attempting to add more than one subscription card at a time.