How do I use an Apple Card to pay for iTunes and App Store?

Currently I have iTunes and App Store payments on my account set up to use a credit card. I would like in future to use my App Card to do this, but when I enter the Apple Card number and information provided (on iOS in Wallet), the card is rejected. I've had no trouble using the same information to make purchases elsewhere, and the description associated with the information suggests that this ought to work: There is no option in Wallet to achieve this either. How do I use an Apple Card as the default card to pay for iTunes and App Store?