map.Event.bind('beforeContextmenu', function(event) {
  var element = document.createElement('div');
  element.innerHTML = 'click me';
  element.style.cursor = 'pointer';
  element.onclick = function() {
    map.Ui.ContextMenu.visible(false);
    alert(event.location.lon + ', '
      + event.location.lat);
  };
  event.add(element);
});

• beforeContextmenu : Context menu is going to appear
• event : Event is the object that sent when the thing happened
• element: Element is the button that inserted in Context Menu
• ContextMenu.visible : Show/Hide context menu