If your studio has put credit on your account, it comes off automatically the next time you book a class or buy a pass. There is nothing to enter.
Open the Purchase page. If your studio has put credit on your account, a green line at the top says how much.
The credit comes off the price at checkout — you do not enter a code or tick anything. If it covers the whole amount, you are not asked for a card at all and the booking completes straight away.
If your credit is smaller than the price, the card page shows only what is left to pay. Your credit is used first, every time.