running Java Script notification when text field is blank

I have a single line text field

I am trying to get a notification at the top of the form to say that the field is blank, if there is data in the field then i want the notification to disappear

I just cant get my code below to work, i keep getting an error

function setDocumentNotification() { 
var ref_field = Xrm.Page.getAttribute("new_reference").getValue(); 
     if(ref_field == "") { 
          Xrm.Page.ui.setFormNotification("Reference field is blank", "INFO"); 
     } 
     else { 
          Xrm.Page.ui.clearFormNotification(); 
     } 
}