the control panel is secure, as is!!
the message is merley an anoyance, th eonly way to remove the message is to use a real cert, that will cost you money, and then the user will have to use the url for the site with the cert, if customers want to suse thierdomain:8443, and not see the message, they also will need a cert.
it is real a waste of time and money in general to remove the warning message, the site is still as secure without the warning messgae, no worries in the secruity dept as is..
cheapest and easiest is to secure your hosting site domina, as you would likely need it anyway, and then tell users to use yourdomain:8443 as the plesk entry point
those are your options