Ah, so as long as you know The Planet's DNS records include either a CNAME or A record pointing
www.domain.com to your server, then use SSH to login to the server as root, use a linux editor (such as nano, pico, or vi) check the following file:
/home/httpd/vhosts/domain.com/conf/httpd.include
Find the section which begins like:
<VirtualHost xx.yy.zz.nn:80>
ServerName domain.com:80
ServerAlias
www.domain.com
UseCanonicalName Off
Where 'xx.yy.zz.nn' is the IP address for the domain. Make sure the 'ServerAlias
www.domain.com' line is there. If it is not, then add that line. Save the changes. Exit the editor. Restart Apache (either via the control panel, or from the command line: service httpd restart)
Note: if you are not using a RedHat type Linux, the command 'service httpd restart' may have to be different for your OS.
Note: Since your DNS is handled 'outside' of the Plesk server, then the Plesk DNS settings shouldn't matter, and this Apache config file should be the place to resolve your problem.
Alternative: If you do not have SSH capabilities, then the other way to try setting it up would be to delete the Physical Hosting Setup for this domain (backup your domain directories and files first), then re-Setup hosting and make sure to select the 'www' option at the beginning.