var exchangeAppointment = new Appointment(exchange);

...

ExtendedPropertyDefinition extendedPropertyDefinition = new ExtendedPropertyDefinition(DefaultExtendedPropertySet.PublicStrings, "keyword", MapiPropertyType.String);
exchangeAppointment.SetExtendedProperty(extendedPropertyDefinition, "value");

var sendModeForSave = SendInvitationsMode.SendToAllAndSaveCopy;
await exchangeAppointment.Save(sendModeForSave);

foreach (var email in command.MeetingAttendeeEmails) {
    exchangeAppointment.RequiredAttendees.Add(email);
}

var sendModeForUpdate = SendInvitationsOrCancellationsMode.SendToAllAndSaveCopy;
await exchangeAppointment.Update(ConflictResolutionMode.AlwaysOverwrite, sendModeForUpdate);

PropertySet YourProperyset = new PropertySet(BasePropertySet.FirstClassProperties);
    var extendendProperty = new ExtendedPropertyDefinition(DefaultExtendedPropertySet.Address, "organizer",MapiPropertyType.String);
    YourProperyset.Add(extendendProperty);
    var folderId = new FolderId(WellKnownFolderName.Calendar, new Mailbox(userName));
    var calendar = CalendarFolder.Bind(service, folderId);
    var calendarView = new CalendarView(start, stop);
    calendarView.PropertySet = YourProperyset;
    return calendar.FindAppointments(calendarView).ToList();