Hmm...I don't think you can do what you want very easily.
I suspect that the web server that listens on port 8443 (for Plesk) would have to have its configuration changed so that it only responds to requests for your hostname instead of for *. I suspect any changes you make would be overwritten following some panel updates.
Why not just not tell your clients that they can access the system usingtheir own domain name? What they don't know, they won't use. Just tell them to use the address you want them to use. And at the end of the day, if you have an SSL certificate installed for your hostname then that will be the only one that won't give an error when they visit it.