To get started, you can add your credit or debit card from your iTunes account to Passbook by simply entering the card security code.
To add a card on iPhone 6, iPad Air 2 or iPad mini 3, go into Settings, open Passbook & Apple Pay, and select “Add Credit or Debit Card”. On iPhone, you can also just open Passbook, then swipe down and tap the plus sign. From there, use your iSight camera to enter the card information or type it in manually. The card you had stored in iTunes is your default payment card, but you can always switch it in Settings.