1. Open the App Store on your iPhone, iPad, or iPod touch.
2. Tap on your profile icon in the upper-right corner of the screen.
3. Tap on "Payment Methods" and then tap on "Add Payment Method."
4. Select "Credit or Debit Card" and enter the gift card number, expiration date, and CVV code.
5. Tap on "Add" to add the gift card to your account.
Once the gift card has been added to your account, you can use it to purchase apps, music, movies, and other content from iTunes. To do so, simply select the gift card as your payment method when prompted during checkout.
Please note that Mastercard gift cards can only be used to make purchases in the country where they were issued. Additionally, some Mastercard gift cards may have restrictions on the types of purchases that can be made. For example, some gift cards may not be able to be used to purchase subscriptions or in-app purchases.