How to setup redirection from site to a site's sub-page? For example, from example.com to example.com/page
The redirect should be specified in main domain index file that would contain a redirection to a required path.
Also, a redirect may be setup in .htaccess file, or in additional directives for a domain in Domains > example.com > Apache and nginx settings
It has to be done by the website developer as this functionality is not related to Plesk: it's a basic webdeveloper task.