What is the best way to validate a discount card online?

We are trying to figure out the best way to re-introduce a discount feature to our website. We have recently undergone a massive project implementing online sales to the site which has left us in a peculiar position.

A high percentage of our customer base uses a certain discount card to book with us, previously they would reference this when calling up to book and the discount applied manually. But now, since the online payment ability has been implemented we have no way to apply a discount to customers who hold this specific card.

The cards themselves are pretty dated and the only way the discount is applied is by checking that the customer is in possession of one.

Are there other ways to validate this feature on a website that I might be missing?