Which FQDN hostname to use for SSL certificate signing request- when using a CNAME record?

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http://serverfault.com – We have a subdomain (https://portal.company.com) that is the alias for a different hostname (defined in a CNAME record). This dynamic DNS hostname (https://portal.dlinkddns.com) resolves to the public (dynamic) IP address of our office. At the office, the router is configured to forward port 443 to a server running a (Spiceworks) web portal that the staff can access from home. Even if the office's public IP address changes, the subdomain will still direct staff to the web portal. Everything works great- apart from the (expected) SSL certificate error staff see when they first connect to the s (HowTos)