You can force the whole site to use www, I don't believe you can force it for a single page. You can change the site setting by going to CMSSiteManager>Settings>URLs & SEO>SEO - URLs>Process domain prefix: and select Always use domain with www prefix.
Then in CMSSiteManager>Sites>(select site) in the General tab change the domain to read www.< yourdomain >.com vs. < yourdomain >.com. Then add an alias with < yourdomain >.com without the www. These two items aren't required but it helps keep things clean.