Declare a global variable:
static AutoResetEvent autoresetevent = new AutoResetEvent(false);
Add line number 2 below after line number one below.
Application.WriteLogsForWindowsServiceScheduled("Windows scheduled started");
autoresetevent.WaitOne();
Do these changes in function ProcessTimerEvent:
if (countdown == 0)
{
autoresetevent.Set();
timer.Dispose();
}