How to redeem an Xbox code.
Online (Recommended)
Go to redeem.microsoft.com
Sign in to the Microsoft Account associated with your Xbox profile
Enter your 25-character code then follow the prompts
Xbox Series X|S and Xbox One Console
Sign in to your Xbox console
From the Home screen, select and open the Store app. (If the Store app is not displayed, go to My games & apps, select Apps, and select the Store app from there.)
In the Store app, move the cursor left or press the View button on the controller to open the side menu
From that menu, select Redeem
Enter the 25-character code then follow the prompts
Xbox 360 Console
Sign in to your Xbox 360
Press the Xbox Guide button on your controller
Select Games & Apps, then choose Redeem Code
Enter the 25-character code then follow the prompts