Specifically, you need to download the Shack App, create an account, and enter the promo code "shackappy" at checkout to get your free burger. The offer is good through February 28, 2017.
The app is meant to offer convenient ordering and shorter pick up times. Through it, you can pick the food you want and choose an available pick up time. The food is made-to-order and should be meant to be ready at your appointed time.
Photo via Shake Shack.