I remember creating a thread in which I asked this. Treeston gave me a 10 line addon that you could expand for other stuff. it was originally meant for rez messages, but I don't have it anymore. if you search for my threads in this forum, you'll likely find it.
You need to edit the code for that. Those addons likely have a "SendChatMessage(X[,Y[,Z[A]]])" somewhere. you always need X, but you don't need Y, Z or A. You need to replace Y with "INSTANCE_CHAT", Including the double quotes.