Subdomains are additional domain names that hosts additional web sites or parts of your web site on the same server without additional domain name registration.

Login to your Plesk Control Panel , under the Website  menu click on subdomain.

Now click on Create Subdomain.

Now need to create your subdomain with require details (Subdomain name, FTP access details and available features) .

 Creating Subdomain with separate FTP access.Click OK to save changes.

Subdomain is created .

 Logout when you are done.