That is just a built in redirect in cPanel, so you won't be able to change it. You can just use the port instead if you want. https://domain.com:2083
Most hosting providers provide FTP or File Transfer Protocol and you can access it in Filezilla or any other FTP client. You can also easily create folder on it. Here's the steps either public_html or www > your site > then right click to create folder.
If you want the index page of your site to redirect to some sub-folder then simply go to 'Redirects' in cPanel, chose Permanent (301) redirect, choose the domain from dropdown (the will be only one if you have 1 domain added in your cPanel), and in 'redirect to' field, add the complete URL where you want it to redirect.