For this request the “Tag” property is available, there you can add a custom value. e.g. an id or a json string if you need to store more than one custom property.
B4X:
Dim Appointment As ASScheduler_Appointment = ASSchedulerUtils.CreateASScheduler_Appointment(0,"Test2","Description",xui.Color_ARGB(255,45, 136, 121),False,DateTime.Now,DateTime.Now + DateTime.TicksPerHour)
Appointment.Tag = 126
ASScheduler_MonthView1.API.CreateAppointment(Appointment)