Excerpt from config.xml:
I made some screen shots which you can look at here:Allows for the creation of Promo Codes that can be passed around to forums or other sites as an incentive to get players to sign up by giving them money, items, etc after they register using the promo code. The promo codes can be set to be used a certain amount of times, unlimited, and they may also be set to expire using DATE format i.e: YYYY-MM-DD. This module also comes with an admin page that will show you the usage of a promo code selected to show which player signed up with that code and when they used it.
Also a youtube video just kind of demonstrating what it does. The video is nothing fancy so be warned haha
https://www.youtube.com/watch?v=a0Y6MQfWdc0
EDIT:
If you get the module and look into the code.php file it will give you directions on how to create a link and maybe a future update will be link generation for the codes much like imgur does for the image links. Another thing I failed to mention is the fact that if you set the code_max_uses to anything less that "0" it will enable infinite uses. I tried to make it as versatile as I could with expiration, and uses. This way you can give some great limited time offers to new players to get them to join.