To create subdomain in Plesk please follow the followinig steps:-
1) Login to your Plesk control panel.
2) Click the "Domains" shortcut in the navigation panel.
3) Click the required domain name in the list.
4) Click on "Subdomain" under "Web sites" category.
5) Click on "Create subdomain "
6) Insert the "Subdomian name" and enable the various services you want on that
"Sub Domain" and click on "FINISH".