How to redeem a gift card with the App Store app on iPhone and iPad
Step 1: Launch the App Store app on your iPhone or iPad.
Step 2: Tap on the Featured tab in the bottom navigation if you aren't there already.
Step 3: Scroll all the way down to the bottom of the Featured page.
Step 4: Tap on the Redeem button.
Step 5: You may be asked to sign into your iTunes ID, do that now.
Step 6: Enter your promo code.