Sometimes, you may want to limit who can successfully obtain tickets to your event, for an event pre-sale, to a private party or a members-only event. You can do that by requiring a code to be entered into the coupon field in order to checkout.
Create your events with ticket prices of at least $0.01.
Manage your event and head to Purchase Settings > Restrictions and Waitlist > COUPON REQUIRED PER TICKET PURCHASED. Click "Disabled" to change the setting to: "Required".
You might consider creating the event as semi-private so that it's not displayed in your main events listing. You may then invite customers/members and send the semi-private event link with instructions to enter a coupon code on the checkout page in order to finalize their order. Your customers will be required to enter a valid coupon code for each ticket in their cart in order to checkout. All other attempts will be turned back, notifying the customer that they must have a valid code per ticket in their cart.