How do i add an apple gift card to pay?

You can only use Apple ID balance by purchasing items. It cannot be refunded or transferred to another account except under very limited circumstances:

Check your Apple ID balance - https://support.apple.com/HT202359 -->

"About unused Apple ID balance

You can't redeem or return Apple Gift Cards, App Store & iTunes Gift Cards, or unused Apple ID balance for cash, except as required by law. If your jurisdiction allows for refund of redeemed gift balances, you can contact Apple Support."