Select the tickets for your event and proceed to the checkout page. On the checkout please select Gift Card as the method of payment. Enter the 16-digit Gift Card number, and the 3-digit security number/PIN. If there is still an outstanding balance once you have entered your Gift Card, you can either enter another Gift Card or select another method of payment (credit/debit card), until the total balance is cleared, and the booking is complete.
More information here: https://www.ticketmaster.cz/feature/darkovapoukazka/faq.en-us.html?language=en-us