I have prepared sample project that demonstrates the approach. It is quite simple:
1) Pass the ID of the appointment that you want to access using the query string.
2) Hook on the DataBound event of the RadScheduler
3) Obtain the ID from the QueryString using Request.QueryString
4) Obtain the appointment using the Appointment.FindByID method of the Appointments collection of RadScheduler
5) Pass the obtained appointment to the ShowAdvancedEditForm method of RadScheduler.