<zk>
<zscript><![CDATA[
EventListener listener = new EventListener() {
       public void onEvent(Event event) {
               alert("click:"+event.name);
       }
}
]]></zscript>

<button label="Test"
onClick='Messagebox.show("Yes and No", "Test", Messagebox.YES+Messagebox.NO, Messagebox.INFORMATION, listener)'/>

</zk>