Once you have an active TenX Card, the interface inside the "Card" button changes; there will now be a "Show PIN" button.
You will then be able to tap on the "Show PIN" button to reveal the 4-digit PIN tied to your card.
For your security, the PIN will only be displayed for 15 seconds before it is automatically hidden again.
Note: this is different to the 4-digit passcode used to unlock your app.
For instructions on how to change your Card PIN, refer to this page.