Well, the obvious answer to this query is “NO”; we can not install SSL certificate on a subdomain (xyz.domainname.com) directly. But like it is not impossible, what you need to do is create the subdomain as a separate host in the IIS. If you purchase a wild card SSL then you can assign that SSL certificate to any host within a domain but again, you will still require a separate host. Hence, you need to separately host the sub-domain on the server and install the SSL certificate.

SSL certificate embeds the DNS name of the host in question and the client browser checks that the hostname embed in the certificate matches the hostname in the requested URL. But the host HTTP header is encrypted when transmitted from client to server, so the server can not use it to determine which site the request should be routed to. Hence, the browser must request a FQDN (xyz.domainname.com).

Hence, the host on which you want to install SSL certificate, even if it is a subdomain, you need to host it separately and have a unique IP address assigned to the site as well as the site should be synchronized with SSL port.