cPanel shared hosting - you can ask your host to do this for you it's easy to do in the back end, but you can't do it from the end-client panel.
However what you can do is make a subdomain and point its document root to /main/public/. You can use discuss, forum, forums, flarum, or whatever else you want.
But if you want it on the main domain, open a support ticket with your host to change the document root to /main/public/. Also make sure you ask them for jailed SSH access if you don't already have it - you need that to install and update plugins.
Make sure you edit the default .htaccess file located in /main/ and uncomment the lines that protect the sensitive files from being accessed by the public.