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?

Source:

These instructions are for iOS 13. It should be the same for versions going forward in the immediate future.

1. Open the Wallet app.
2. Tap your Apple Card.
3. Tap the ••• button in the upper-right corner.
4. Scroll down and tap Make Default at Apple.

These steps make your Apple Card the default payment method for all of your transactions with Apple, including app purchases from the App Store, Apple TV subscriptions, iCloud storage, your Apple Music plan, and even AppleCare if you choose to pay per month.

You can double-check that Apple Card is your default by going to "Settings -> Your Name -> Payment & Shipping". Your Apple Card should be listed as the first payment method.

If you have an Apple Watch, you can check to see if the Apple Card is its default payment method by opening the Watch app and tapping "My Watch -> Wallet & Apple Pay -> Default Card".