In the event dashboard, click on Registrations.

From the sidebar menu, select “Setup Coupons”

Select the type of coupon you want to create and click next.

Enter the coupon details and click on the Advanced section.

Expand the Advanced section and click the "Apply only to some tickets" field.

Choose the tickets for which this coupon can be applied.

Once set up, click next, which shows the summary of the coupon to be set up. Click save to create the coupon. 

How to set up coupon codes to apply only to specific tickets?