Follow these steps to redeem your iTunes Gift Card:

  1. Open iTunes or the App Store
  2. 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
  3. Enter the iTunes Gift Card which you can find under My Withdrawals by clicking on "Show Code"
  4. The balance has now been added to your iTunes account

Note: You can find the official instructions here.