I've done this twice and it works. Redbox is giving away a free movie every Wednesday in the month of March in addition to the free movies they give away every Monday.
The movie is available today only at one of these locations.
The free movie code is JBG123. The code expires at midnight
The process: Go online and see if the movie that you want is available at the nearest location to you. DO NOT RESERVE ONLINE (If you reserve online you will have to pay)
Go to your nearest redbox and order your movie
At the end of your transaction be sure to select use promo code. You will have to swipe your credit card but you will not be charged.
Be sure to bring it back by 9PM Thursday or you will be charged $1