Discount codes are created inside of Shopify, not the theme. The theme has no way of seeing or displaying any of these codes. You can advertise the code or embed a shareable discount code link into the announcement bar or other places. If you would like a pop up to appear you can try looking through some of the Discount code related apps.
Whilst we are always happy to support our themes and provide guidance on how they work by default, for any topics that are related to an app, the fastest way to resolve this is to speak directly with the third-party app developer who created the app.