How add remind before END date of appointment?

Question

I need add remind about appointment before end date, not start date.

This appointment's creation

private async void AddFavoriteTenderEndDateToCalendar(MyEvent event)
    {
            var appointment = new Appointment();

            appointment.StartTime = new DateTimeOffset(event.StartDate);

            appointment.Duration = event.EndDate - event.StartDate;

            appointment.Subject = "blahblah";

            appointment.Reminder = TimeSpan.FromDays(1);

            var appointmentId = await AppointmentManager.ShowEditNewAppointmentAsync(appointment);           
    }

Appointment have not property EndTime and instead I calc duration appointment.Duration = event.EndDate - event.StartDate;

How can I add reminder before event.EndDate?