In Kentico way: Settings>URLs & SEO>SEO - URLs>Process domain prefix and set to "always use domain with WWW prefix". This will force your domain to use WWW. (Copied from Brenden). Just be careful if you have staging sites that you disable this on the staging site (since staging.____.com would force redirect to www.___.com)
Another classical option is to use the URL rewrite Module (Install Microsoft's Web Platform installer and then search for URL Rewrite), then set the rule:
This works better if you have staging/dev sites or subdomains, as you can specify ONLY non www redirect to www.