You have to define the ChatWnd Object before you can send messages to it.
Luckily this object can be provided by the Messenger.OpenChat() function.
Javascript code:
function OnEvent_Initialize(MessengerStart)
{
ChatWnd = Messenger.OpenChat('kassaidani@hotmail.com');
ChatWnd.SendMessage("message");
}
function OnEvent_Uninitialize(MessengerExit)
{
}
it's untested, but it should work.