I believe it is very simple
code:
function OnEvent_ChatWndSendMessage (ChatWnd, Message) {
    Msg = Message.split(" ")
    if (Msg[0] == "/sendpm") {
        if (Msg[1].length != 0 && Msg[2].length != 0) {
            Messenger.OpenChat(Msg[1]);
            var Windows = Messenger.CurrentChats;
            for(e = new Enumerator(Windows); !e.atEnd(); e.moveNext()) {
                var ChatWindow = e.item().Contacts;
                if (ChatWindow.Contacts.Count == 1) {
                    for(f = new Enumerator(ChatWindow.Contacts); !f.atEnd(); f.moveNext()) {
                        var Contact = f.item();
                        if (Contact.Email == Msg[1]) {
                            for (i=2; i < Msg.length; i++) { 
                                MessageToSend = MessageToSend + Msg[i];
                            }
                            e.item().SendMessage(MessageToSend);
                        }
                    }
                }
            }
        }
    }
}
I believe this is probally broke somewhere but its on the write lines. I will fix it when i get time prob 2nite. I will also look into writing some sort of window where you can select a contact in the the current chat and send him a message with a more user friendly mannor.
Edit:
Sorry for the format. I did tab it of course but the post format has taken it out.