Use an Apple Gift Card in Music on Mac
If you received an Apple Gift Card, iTunes Store Gift Card, or promotional code, you can use it to buy songs and other items from the iTunes Store.
Choose Account > Redeem, then follow the onscreen instructions.
You don’t need to enter credit card information to redeem only the amount of a gift card.
To check your balance, choose Account > View My Account, sign in (if necessary), then go to the Apple ID Account section.
Gift cards are valid only in the country or region where they were purchased.