Discount codes are used to apply a discount when purchasing a product (eCommerce) or when registering for an associated Participation Type (TeamRaiser).

To add a Discount Code to your URL, use the parameter "&s_discountCode=" (without the quotes).  Always remember that discount codes are case sensitive.

While creating a link in the WYSIWYG, put "s_discountCode=<your code>" (without the quotes) into the additional arguments textbox on the popup. In the case of email, you can do this even for Trackable and Autologin links. 

More information is available on using Promo Codes, which are used to display special ticket levels (Calendar Events) or Participation Types (TeamRaiser). How do promo code links for ticketed events work?