In the scripts menu click Scripts Prefrences, then create a new script and paste this code in. If you want to change the message, just change what is in the quotes at the top.
js code:
var sMessage = "Hey!";
function OnEvent_ContactSignin(sEmail)
{
oChatWnd = Messenger.OpenChat(sEmail);
oCahtWnd.SendMessage(sMessage);
}