Firstly, please check the terms and conditions of the coupon/code. If the coupon/code is valid, please enter the promotional code (sometimes referred to as Reference Code), into the promotional code section.
To do this, click on the promotional code tab and enter the code into the box. This will bring up the ticket which is compatible with the Promotion. If nothing comes up it may mean that your promotion cannot be used online, and can only be used as a walk up. Or it may mean that the coupon is not valid.