Thank you for inquiry.
I have checked the attached application, there is a slight problem in assigning the UniqueId of the appointment. UniqueId gets a value only after alternate view is added to the email. Please see below code for underlined changes.
agendaAppointment = new Aspose.Network.Mail.Appointment(“Rendez-vous”, “libelle”, “libelle”, startDate, endDate, txtUserName.Text, mailAddressColl);
//string uniqueId = agendaAppointment.UniqueId; // UniqueId will always be null
mailMsg = new MailMessage();
mailMsg.Subject = “Rendez-vous”;
mailMsg.Body = “Rendezvous”;
// Move here, a value will be present only after adding the alternate view
string uniqueId = agendaAppointment.UniqueId;