Claudia, I've always grabbed the complete URL from the browser toolbar. Just make sure you're on the home page rather than in assignments or modules or some other tool, or the URL will capture that layered destination rather than the course as a whole. Once I have the URL, I can go to the course where I want to add the link and add it in the same way I would any link in the content. You can even use the redirect tool (from Settings) if you want to add it directly to the navigation menubar.

One caveat -- the participants need to be enrolled in both courses, or the they won't be able to enter, even if they have the link.

Cindy.