If you were issued a coupon code to be used towards a future event, you can find this code by logging into your account and selecting the "Your Coupons" section. The opportunity to use a coupon code will appear in the registration process.
If you've received a coupon code from another source such as via email or text, you will be able to input this coupon code prior to the payment step in the registration process.
For more info: https://support.smoothcomp.com/article/272-how-to-use-a-coupon-code