You cannot at the same time use a domain name for a website AND protect it by an SSL certificate for the port :8443 login page.
Could you please describe what the underlying issue is? You can easily protect the Plesk login page with a free Let's Encrypt certificate. You can also create a custom URL for the login page and protect that custom URL with a free certificate. If a user chooses to address that page through the user's domain that is used for a website otherwise, he can do that, but then he'll not get the correct certificate. As there is no need to open the login page through a "wrong" URL this should not pose an issue.