Put money in my apple id balance
How do i add funds to my apple id balance in order to purchase more storage
iPhone 8, iOS 13
Buy/redeem Apple Gift cards to the account.
App Store & iTunes Gifts - Resend
Or
Apple ID - Add funds to your ID This won't work if you are a member of Family Sharing and not the organizer.
If you are going to get an iCloud storage subscription, you will also have to have a credit card on your account.