macOS Mojave 10.14

Use an iTunes or Apple Music Gift Card on Mac

If you received an iTunes Store Gift Card or promotional code, you can use it to buy songs, videos or audiobooks from the iTunes Store.

Ask Siri. Say something like: “Redeem an iTunes Store gift card.” Use Siri on your Mac.

Set up an Apple ID (required before you can redeem anything in the iTunes Store).
In the iTunes app on your Mac, 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.

Your balance appears in the upper-right corner of the Store window.

You can only use a gift card in the iTunes Store of the same country or region in which it was purchased.

Helpful?

iTunes User Guide