Setting Domain name for FIRST site in MULTISITE environment

Is it possible to set a domain on the first site in a Multisite environment?
I currently host in Azure, my current Siteurl has <servername>.azurewebsites.net as the domain

I've configured the appropriate domains for all the sites I want to host, all of my sub sites work correctly with Domain Mapping. However, the root site, i cannot setup a domain name on...I'm guessing this is by design. if not, can someone help me with setting the domain mapping for a root site?
My only other option is to setup another sub site, and map the domain to it...which I can do just don't really want to.

thanks
john