AWS Cloudformation ouptut

I am creating a cloudformation template for users which will create instances and give an application end point and user name and password . I am able to achieve unique application end point with respect to instance ID but I am not able to generate a dynamic username . Could any one help with the function that should be used to achieve different username on every new attempt. Below is the sample.

"Outputs" : {
"EndpointURL" : { 
  "Description" : "Application End Point",
  "Value" :  { "Fn::Join" : [ "", [ "http://", { "Ref" : "DNS****"}]]}
  },
  "Username" : {
   "Description" : "Dynamic Username required",
   "Value" :  "**********.com"
  },    
  "Password" : {
   "Description" : "Password",
   "Value" :  "Fixed"
  }
}

}