To enable subdomain with WWW prefix, please follow the following steps :-
1) Login to your Plesk control panel.

2) Click on "Websites and Domains".

3) Click on the DNS Settings.

4) Click on [Manage] in the front of that domain name only.

5) Click on "Add Record".

6) Select CNAME from the drop down menu.

7) Add the following detail as marked in RED.

8) Once you have made the changes Clik OK and then click on "UPDATE" to apply the changes.
