These are ‘coupon code’ based discounts. Anyone who enters a live, valid code at checkout will receive the discount, so be careful. These codes are valid for ALL of the event distances for your current event (codes will be invalid after you renew your event).
Navigate to coupons & invitations, and click the “coupon codes” tab:

Complete all fields & click “add”.

All LIVE codes can be seen under the “Live Coupons” section. Just click to expand the section:

You have the ability to edit or delete the code once it has been created:
