To add a discount to a registration that has been already been submitted, go to Registrations and click on the row of a registration that needs to have the discount added. The discount can be applied to a registration that had selected one time, recurring, or offline payment.
Click on Apply Discount.
A side pop-up should appear with a drop-down menu. Click on the down arrow to get the discount selections. Click on the discount code to have it applied to the registration form.
To modify a discount that has been applied to a registration, click on the pencil icon and follow the same steps as above.