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.
There is also a way for an organiser to apply retrospective discounts at their side. They can use this for example if they can see you've applied to one or more of the events. In this case you would not be supplied with any code - you may need to give them a little prod though it they forget to orchestrate this.
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 and pay the rest on the given balance due date.