To obtain your code for a free Burger King sandwich, take a look at the back of your receipt. If it says, «Want a Free Whopper? You can then complete the survey. When you're done, you'll be given a unique validation code to write on your receipt. When you purchase a small, medium, or large drink and any size fries, you can use your code for a free Whopper Sandwich, Original Chicken Sandwich, or Croissan'wich Sandwich.