Open the Wallet app on your iPhone
Tap the blue plus (+) icon in the top right corner of your screen
Tap Continue, then tap 'Enter card details manually'
Enter your name as it is printed on the card and the full card number in the fields provided, then tap 'Next'
In case you are asked to verify your information to continue, call the number that appears on your screen (833-263-6685) and the card processor will be able to help you. They are available Monday through Friday, 10 AM to 6:30 PM EST.
Once you've added and verified your information, your card is ready to use with Apple Pay
For more information about Apple Pay, including setup and usage, see Apple's support site here.