code:
function OnEvent_ChatWndReceiveMessage(ChatWnd,Origin,Message){
if(Origin !== Messenger.MyName) ChatWnd.SendMessage(Message);
}
should work but Plus! has a limit on ChatWnd.SendMessage so it might stop working after 15 messages sent in a period of time
.
it's a really simple script to do, just read the scripting documentation and you'll learn how to do more