A promotional code is offered by event organizers and can be used to get discounts on tickets, or access hidden tickets.
To Apply a Promo code:
Click on "Do you have a promotional code?"
Enter your code and click "Apply"
Select the ticket quantity and click "Register"
Note: The organizer may share the registration link with the promotional code already applied in it, in this case, you don't need to enter it again just select the ticket quantity and proceed with the registration steps.