Subdomains in Plesk
From Lunarpages Web Hosting Wiki
A subdomain would be a domain name that has a prefix in front of it representing the folder that it came from. For example, If you setup yourdomain.com/blog/ as a subdomain the new address for it would be blog.yourdomain.com.
How to Setup Subdomains in Plesk
The first thing you must do is login to your Plesk control panel. Once you have made it to the front page, click on the domain name related to the subdomain you wish to create. Now, click on the "Subdomains" link.
Here you will see all the subdomains you have created as well as a link to create a new subdomain. Click on the "Add New Subdomain" icon to create a new subdomain.
In the text box for subdomain, be sure to put in the name you wish to use for your subdomain (such as example.yourdomain.com, example is what you would put in the text box).
You will also need to put in unique attributes for your FTP login and password that will be just for this FTP account that goes to the subdomain you are creating.
Under Microsoft FrontPage support, you can enable or disable this option for your subdomain.
Then under "Services" you can put in a check mark box for each service you wish to have supported under the new subdomain.
When finished, double check the values you have entered and hit the "OK" button. Your subdomain has now been created!