Plesk has the tools you need to create a staging environment for your website. Web professionals consider this kind of workflow a "best practice" for building out a site; it solves a lot of problems Web developers encounter including simplified relative paths and fully functional site previewing.
Note: If your domain name and hosting account are not in the same ITEGY account, you will need to create a new A record for your subdomain that points to your hosting account's IP address (more info).
At the most basic level, you will develop your website using the staging site — you'll modify its files instead of your main site.
Once you've finished developing the site using the staging environment, you'll copy the staging site to your main site(more info). Voila! You've now got the new version of your website visible on your main site.
When you're ready to develop your main site further, proceed copying it to the staging site (overwriting its old files) and repeat this process.
It is important to note, though, that staging sites do not copy databases. You can, however, create copies of databases by going to the Database page in Plesk or export/import your database's contents through phpMyAdmin.