Please follow step by step guide to setup this on your store :
Step 1. Create a Discount Code from Shopify. Click here to learn more about how to create discount codes.
Step 2. Login to AiTrillion.
Step 3. Create a Newsletter Popup. How to Create?
Step 4. While creating the newsletter popup go to the content tab and put the discount code in the Sign-Up Success Message.
Step 5. Click on the Save button to save the popup.