Redirect domain

Hey, one more ex-Cpanel user with a question =)

My website behaves different according to if the website access is made through www.domain.com or just domain.com. The difference is that the cookies doesnt work with both at the same time and that causes the need to login again if it's accessed with or without the www.

The way i found to fix this was redirecting http://mdown.org to http://www.mdown.org on the Cpanel settings. Like this:http://img401.imageshack.us/img401/295/ter.png

So, my question is, where can I find a way to do the same in Virtualmin? I'd really appreciate if you could give a hand on this matter.

Thanks, Solutho