The easiest way is to download the
Passport app for iOS. When you log in for the first time, you will be prompted with an "Add to Apple Wallet" button. Tapping it will walk you through the process using iOS push provisioning, which allows your Passport card to be added directly and securely to your Apple Wallet without needing to enter your card details manually. You will just need to accept the "Terms and Conditions" and select the device(s) you want to add the card. Lastly, you will need to verify your card via a SMS confirmation text.
If you have dismissed this initial "Add to Apple Wallet" notification, you can access it by tapping on the Wallet button on the bottom of the screen. Then tap on "Card details" and then tap on "Add to Apple Wallet".