Accessing a non-existing domain in Plesk that resolves to the Plesk server IP address shows Plesk login page instead of the Plesk default page
The option All domains and subdomains that resolve to the server IP address but are not used for hosting is selected at the Tools & Settings > Customize Plesk URL page.
Apply the following steps to restrict access to the Plesk login page to port 8443:
Select the option No custom URLs. Only https://<server-IP-or-hostname>:8443 at the Tools & Settings > Customize Plesk URL and click OK to apply the changes.