On the payment screen during checkout, click the Add Gift Card section and enter your gift card number and PIN. All Ticketmaster Gift Cards have a PIN number. If your order totals more than the balance of your gift cards, you'll also need to input your credit or debit card details.
You can only use Gift Cards on new orders.
If you don't see the button to apply a Gift Card, then you can't use a gift card for that event.