A very common scenario when using RadCalendar for Xamarin is the usage of custom appointments. When you create a Custom Appointment class you gain the ability to append additional properties to the base Appointment class.
From R3 2020 Release you can customize the Scheduling UIs. In addition you can modify the Scheduling UIs adding custom UI within you can change the property/data in the custom appointment.
We have a sample demo in our Telerik Sample Application where you can check how property from the Custom Appointment class is added to the Scheduling UI - AddAppointmentView screen. For this purpose you need to customze the AddAppointmentView ControlTemplate.