How to Retrieve Custom Value from Lookup using Javascript

Hi all trying to Retrieve Custom Field Value from Lookup but it showing undefined 

Here am Retrieving ParentcontactId  & AlertNotification(Custom field) from Contact 

function = LeadAlertNotification(){
debugger;
var entityName,entityId,entityLabel,ParentContact;
LookupContact = Xrm.Page.getAttribute("parentcontactid").getValue()[0].name;
if (LookupContact !=null){
var AlertNote = Xrm.Page.getAttribute ("new_alertnotification").getValue(); 
Xrm.Page.ui.setFormNotification(AlertNote, "WARNING");
}
else {
Xrm.Page.ui.clearFormNotification();
}
}

Please Give Some Suggestion 

Thank you 

Regards

Sk