I am usually given a discount/free stand but have no promotion code?
Simply call the organiser concerned straight away
If you are entitled to a discount, they will provide you with a promotion code. This is simply entered at the check out in a box, at the top of the order. Be sure to click on 'Apply' next to the box. The promotion code will be applied to your total, then click the 'Apply Now' button again under the order.
If there is a deposit scheme, don't forget to select this option on the order, if you wish to just pay a deposit at that time.