Don't forget the "return '';" otherwise the message'll be send without "qweqwe :" too.
code:
function OnEvent_ChatWndSendMessage(ChatWnd,Message){//when you send a message
ChatWnd.SendMessage("qweqwe: "+Message)//prefix your message with qweqwe:
return "";
}
or
code:
function OnEvent_ChatWndSendMessage(ChatWnd,Message){
if(Message.charAt(0)!="/") return "qweqwe: "+Message;
}