How to map a domain to the main site of a MU install

I have a MU install in a subdirectory "/public_html/anvoy" and I want to point my anvoy.com domain to it. I setup the Addon Domain and the site loads when I enter anvoy.com, but the URL in the browser changes to agileappdevelopment.com/anvoy, because agileappdevelopment.com is primary domain for the hosting account. How can I change this? I need this so that my MU site show as "anvoy.com/site1" not "agileappdevelopment.com/anvoy/site1" Thanks!