To purchase a promo (or "secret") ticket on Pelago, you need to first download the Pelago app from the Apple App Store or Google Play Store.
Once you have installed and logged in to Pelago, scroll through the Explore screen to find the event you want to buy tickets to. Click on the event image to access the event details page.
The tickets section of the page will include a field where you can enter your ticket access code. Type your code in and press "Book Now".
The promo ticket will be displayed at the bottom of the Purchase Ticket scree 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 or entering your credit cards details manually.