cPanel already have an internal proxy that redirect you through the port 80 or 443 usually open to access any website.
If you are behind a firewall you can access cpanel, webmail, whm or webdav using the following url format:
https://cpanel.yourdomain.com
https://whm.yourdomain.com
https://webmail.yourdomain.com
https://webdav.yourdomain.com