To enable subdomain with WWW prefix, please follow the following steps :-
1) Login to your Plesk control panel.
2) Click on "Subscriptions".
3) Click on "Manage Hosting".
4) Click on "Websites and Domains".
5) Click on the DNS Settings.
6) Click on [Manage] in the front of that domain name only.
7) Click on "Add Record".
8) Select CNAME from the drop down menu.
8) Add the following detail as marked in RED.
9) Once you have made the changes.
Click OK and you get in Green Bar {Information: The resource record www.subdomain.gwdemo.com. CNAME gwdemo.com. was created.}
10) Click on "UPDATE" to apply the changes.
Information: Changes made to DNS records were saved.
