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).
Note: 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?