Open the booking dialog for the relevant booking.
Enter the promo code or gift card code; the booking will update automatically.
Without a connected register:
Click the green Check In button.
If prompted, confirm payment by clicking the button again.
The booking is now checked in.
With a connected register:
Click the green Send to Register button.
Status will change to Waiting for Register.
Complete the transaction in the register.
Once payment is confirmed, the booking is automatically checked in.