Software Deployment

I am creating a transform for Diskeeper 2007. To supress the CD key window, they advised to add LICENSENAME="c:\xxx\license.dal" to the command line. But, is there a way to add that as a Property (In Application Configuration - Setup Properties) and give it a value that points it to the install dir for the msi, you know basically a %path% type value. I think I can just add LICENSENAME as a new property, but have no idea how to add the value. Thanks!