Items API Events
button:[name]:clicked
Fires whenever a custom region button or custom dialog button is clicked.
You would want to be notified about this event firing, because you will naturally have some code that you want executed when your custom button is pressed.
Note The name in the event is the name/id given to the custom button, for example a button referenced as 'myCustomButton' will have an event called 'button:myCustomButton:clicked'.
Examples
itemsApp.on('button:myCustomButton:clicked', function () {
console.log('This code executes when myCustomButton is clicked.');
});