Custom Actions
If you supply a non-recognized action name, that name is treated as a custom action.
The Window then renders k-icon and k-i-actionname CSS classes for the action but does not automatically attach a click event handler to it. While the Kendo UI stylesheets provide a "custom" icon for custom actions, you can use any of the icon names. To capture and handle the click events, follow the standard approach.
$("#window").kendoWindow({
actions: ["Custom", "Minimize", "Maximize", "Close"],
title: "Window Title"
}).data("kendoWindow").wrapper.find(".k-i-custom").click(function(e) {
alert("Custom action button clicked");
e.preventDefault();
});