The root domain's A record should point to 54.209.77.18.   Configuring your Root domain (example.com) in this way will automatically redirect those requests by appending www. to the root domain (www.example.com).

** Please note that we recommend against making these root domain changes in on your Domain Controller/Active Directory Server. **  

After this record is set up you may follow the directions in found here: How do I create a subdomain redirect?