From the Microsoft Store:

To claim the game open a Windows 10 or 11 device and search for "Microsoft Store".
Ensure that you are signed into the Microsoft Store under the account you wish to play DOOM Eternal then select the account button in the upper right hand corner, next to the search bar.
A menu will open with the option to "redeem code or gift card".
Select that option, input the 25 character code, and click next to link the content to the account.
From a Microsoft account: