Shoutbox

You should not replace the 'Email' parameter

When the function 'OnEvent_ContactSignin' is called as an event, it passes one parameter named Email, so you know who signed in...
If you want to do this for only one person, you should check if the Email parameter is the same as the email of the person you want

function OnEvent_ContactSignin (Email) {
   if (Email == "example@hotmail.com") {
      var Contact = Messenger.MyContacts.GetContact(Email)
      Messenger.OpenChat(Contact)
   }
}