If you didn’t add the ‘www’ prefix at the time of setting up the domain, even adding it back in later does not seem to work.
To work around this problem, firstly – turn on the ‘www’ prefix again within the Plesk control panel.
Go to Plesk CP > Domains > yourdomain.xxx > 'Domain Administration' drop-down menu > Rename domain. It will force IIS to configure www.yourdomain.com hostheader for your domain.
Also, make sure your www.yourdomain.com resolves to the same IP adress as yourdomain.com.