Using an IIS server with multiple sites behind an application loadbalancer

I am trying to put some sites on an iis server behind a loadbalancer on aws.

The loadbalancer has ssl certificate from acm.

To have the site reachable on the configured subdomain, I have to bind both the loadbalancer and the subdomain url to the site, but I have more than one site on that server, meaning...I cannot bind the loadbalancer url to more than one site at a time. Why do I have to bind the loadbalancer url to a site before it works, I used to believe binding it to the subdomain url was sufficient? If I bind only the url of the subdomain to the site, it would be unavailable...please what configuration or steps do I take in cases like this?