To enable WWW for your website please follow the following steps:-
1) Login to Plesk control Panel.
2) Click the "Domains" shortcut in the navigation panel.
3) Click the required domain name in the list.
4) Click on "Domain Administrator Access".
5) Click on check box for WWW in front of "Domain name".Click on "OK" once you
have made the required change.
Note:- Or you may also enable this by adding the following entry in your "DNS
Settings" under "Website" category in Plesk control panel.
" www.your-domain.com. CNAME your-domain.com."
Click here to view "Name server default restore" article