If I'm understanding you correctly you just want to give the client a messagebox? (by the way, that's default. to do it to all you would loop through the clients, unless there's a function I don't know about.)
commandtoclient(%client,'MessageBoxOK',"Your title goes here.","Your message goes here.");
First argument is the client target, second is the command to give to the client (in this case, MessageBoxOK), 3rd is the title, and 4th is the message.