onEventClick

The event occurs the user clicks on one of the events. getFieldValue('controlId.changedEvent') or eventArgs.details can be used to determin the clicked event

app.startApp({
      appName: "appointmentDetails",
      id: eventArgs.details.id, //eventArgs.details provides the clicked event object
      startMode: "modal"
   });

Google Calendar

Here are some usecases for event clicks you might find helpful. In Case you are using a google calendar, you could open the calendar event in a new window.

    window.open(eventArgs.details.url, '_blank', 'width=700,height=600'); //this is our custom operation. we will open the event in a new window.

Or you can return false at the end of the event source code, which means, that you did not handle the event by yourself and there for to execute the default behavior.