Help With Domain Mapping

I have set up domain mapping on my multisite. I have my multisite set up using subdirectories. I cannot figure out how to get my forwarding domain to show in the url. For instance, my multisite main domain is , I have created a subsite, I have a domain of that I am forwarding to the subsite. When I type in www, it points to to the correct subsite but the domain name shown is the subsite not my How do I configure it where if you enter it points to my multisite but still remains in the url, not switching to the subsite url?