electronic gift card how to redeem it for apple tv subscription- it doesn't have an option in adding billing info for that form of payment

Apple subscriptions get charged to the primary payment method that is linked to your Apple Account.

To use Apple Gift Cards with this, redeem these cards to transfer the amount as account balance for your Apple Account. Account balance will always be used first, before other payment methods (in order to avoid a build-up).

How to redeem your Apple Gift Card or App Store & iTunes gift card - Apple Support Add money to your Apple Account balance - Apple Support

Check your Apple Account balance - Apple Support

What you can buy with your Apple Gift Card or Apple Account balance - Apple Support