Follow these steps to redeem your iTunes Gift Card:
- Open iTunes or the App Store
- Depending on whether you use iTunes or the App Store:
- In the App Store, sign in with your Apple ID, click your name or photo in the lower-left corner, then click "Redeem Gift Card"
- In iTunes, choose "Account" > "Redeem" from the menu bar at the top of your computer screen or at the top of the iTunes window, then sign in with your Apple ID
- Enter the iTunes Gift Card which you can find under My Withdrawals by clicking on "Show Code"
- The balance has now been added to your iTunes account
Note: You can find the official instructions here.