title: Facebook Messenger :How to show a greeting message when start conversation
body: <p>I want to show a greeting message when start a conversation on Facebook Messenger ( before any user input ) . How to do it? <br>
  The problem is : My code is for server by NodeJS ,and it only trigger when a message is send to server ,which mean it only be triggered when users send something.<br>
  Facebook messenger doesn't send anything when you press button "Message".
  I have check this link Messenger Greeting , but it only shows when a new user starts chat ,but i need show it when starting conversations</p>