Limit of 75,000 free sandwiches during offer period. The free item will appear several minutes after sign up under the “perks” tab in the app. And the free item does expire two weeks after sign up.
Terms say it is good for 1 Originals, Skinnys or Flats sandwich up to $8 value, but it seems it can be used on any item and will take off up to the $8 limit. So, you could get a larger sandwich, for example, and pay the difference after discount.