Q: Link Apple Pay to iTunes account

Hi

When setting up Apple Pay, you can add a supported card that is already on file with iTunes by entering that card's security code (as prompted after tapping on "Add Credit or Debit Card" within the Wallet app on iPhone):

Set up and use Apple Pay on your iPhone or iPad - Apple Support

However, you cannot add your iTunes account to Apple Pay or use Apple Pay as a payment method for iTunes:

Payment methods you can use in the iTunes Store, Mac App Store, App Store, and iBooks Store - Apple Support