Login to your Plesk control panel at http://yourdomain:8080/ and Under Websites and Domains click on the Add New Subdomain button.
Fill in the Subdomain name field and the Document root field with the appropriate information and click OK
You then need to edit the DNS records for your domain name and add an A record to point the subdomain at the IP address of the hosting server.
You can see the IP address of the server on the Websites & Domains page in the control panel.
