You can make a ticket type hidden for all ticket buyers unless they have the access code to unlock the ticket. You might for example want to send out an access code to all previous customers who can access the ticket type only if they have the correct code. To make a ticket type hidden just enter the advanced options by click on the Cog icon when adding your tickets.
You can then set-up a Promo code to reveal the hidden tickets by following the instructions
here.
When the user enters a valid promo code they can then access the hidden ticket type on both the website and app.