How to use apple gift card as a payments
How to use apple gift card as a payment method. No option to add it under Add Payment screen on iphone
See the following Apple articles:
Add money to your Apple ID balance
Add money to your Apple ID balance – Apple Support (UK)
How to redeem your Apple Gift Card or App Store & iTunes gift card
How to redeem your Apple Gift Card or App Store & iTunes gift card – Apple Support (UK)