The "Host" header distinguishes between various DNS names sharing a single IP address, allowing name-based virtual hosting. While optional in HTTP/1.0, it is mandatory in HTTP/1.1.
However, while the UI is not letting you doing it, it does not mean you cannot! And actually, it is really easy to do as long as you can open a command prompt. The trick is to use the magical Appcmd.exe tool, which allows you to do almost anything in IIS. The following command line shows how to add a https host header for for a web site named "MyWebSite" with a url "mywebsite.mydomain.com"
appcmd set site /site.name:"MyWebSite" /+bindings.[protocol='https',bindingInformation='*:443:mywebsite.mydomain.com']Read Configure a Host Header for a Web Site for more details
