How do you put a voucher code in for Vue cinema online?

I have a gift card, and I want to book some tickets online, but I don’t really get how to do it : /
Please help x

After you’ve selected a movie and showing, it will ask you to select how many tickets of each type (adult, student, etc) on this screen, there is a box to type in a “promotional code”. This is where you type the number from your gift card.