First, go to your event and then to the Features tab:
Then click on the New Feature button:
That will open up the New Feature box. Give your voucher a name, a category, a description, and indicate the gender(s) and age group(s) for which it will be available.
Set the Maximum Quantity a team can use (usually 1) and the Quantity Available (1 for a single use voucher, or set the total number available for use), the amount (a negative amount for a discount) and then the voucher code that the team will use to get the discount. Check the 'Active' button to make the voucher available.