Create Domain Alias for your customer:

2.)  Click on Your Domain Name Ex: avinash.com  : you can see the below Interface used for editing your Domain properties.

J.) DOMAIN ALIASES : Its is used as an alternative name for your Domain name
1.) Click on Domain Aliases :
 

2.)  Click on ADD  and add a new domain alias to this Domain Alias name without  the 'www' and SAVE it ..

 
3) after that you will get this detail.