You can add the card using two ways:

1. Through the ila app

Using the ila App to add the card to the wallet using a click of a button and will be redirected to the wallet to accept
T&Cs

2. Through the Wallet app

Open the Wallet app.
Tap the "+" sign in the upper-right corner.
Enter your card's security code to add your Debit Card affiliated with your iTunes account or select "Add a different card," and use your iPhone camera to scan the card information.
Apple Watch

Open the Apple Watch app on your iPhone.
Tap "Wallet & Apple Pay" and select "Add Credit or Debit Card "
Enter your card's security code to add your Credit or Debit card affiliated with your iTunes account Or, select "Add a different card," and use your iPhone camera to scan the card information.

*For a list of compatible Apple Pay devices, see support.apple.com/km207105