If the organizer has set-up promo / secret tickets accessible with a code then the tickets section of the event page on web or on the Pelago app will include a field where you can enter your ticket access code. Type your code in and press "Book Now".
The promo/secret ticket will then be displayed on the Purchase Ticket screen with a distinct icon. Select the quantity of tickets you want to buy and press "Continue". You can then purchase your tickets by either scanning your credit card (on app only) or entering your credit cards details manually (on app or web).