how do I force www. in domain name?

hello

I have a domain roapm.com. And I am needing to force it to http://www.roapm.com when people are visiting the site. I have the domain A record pointed to my server IP, as well as the www. as a cname pointed to the IP. (see screenshots) I cant get the site to force to www. I have tried adding the command in the htaccess file but with no luck. It just keeps sending me back to roapm.com and not http://www.roapm.com. Any ideas? I can send you FTP info if that will help. I have enabled support staff access on the site roapm.com

thank you!
Cody