Thank you for the update.
Programmatically Selecting Date feature:
As we mentioned in our previous update, programatically selecting the date support is not available in Xamarin Forms. We have already logged the feature report for the same and we will include this feature in any of our upcoming Volume release.
Requirement in Sample level:
Your requirement of adding the Appointment in the Selected date, when clicking the button can achieved in the sample level. Using CellTapped event we can get the Selected DateTime in the DateTime argument, by setting this DateTime as the Appointment DateTime in the Button Clicked event (editor) your requirement can be achieved. We have prepared based on this scenario, please find the below sample.
Please let us know, if you have any further query on this.
Subburaj Pandian V