If you are an individual customer and have BT Pay installed on your bank's registered telephone number, you can add your cards to BT Pay even before you take possession of them.

If you have a corporate card, it can only be added to BT Pay after you pick it up from the establishment by manually entering your card details.
Adding is quick from the top-right of the screen: just press the '+' symbol and choose the card you want to add.

You can then use it to make payments or withdraw cash from BT ATMs by choosing 'Withdraw cash'. 😉