Telerik UI for UWP RadCalendar How to use commands

I am not sure how to use The RadCalendar comands to update a property in my view model. I follwed this sample from Telreic.

https://docs.telerik.com/devtools/universal-windows-platform/controls/radcalendar/commands/celltap

I created a class as stated:

  public class CustomCellTapCommand : CalendarCommand
{
    public CustomCellTapCommand()
    {
        Id = CommandId.CellTap;
    }


    public override bool CanExecute(object parameter)
    {
        CalendarCellTapContext ctx = parameter as CalendarCellTapContext;
        var date = ctx.CellModel.Date;
        if (date > DateTime.Now)
        {
            return false;
        }

        return true;
    }

    public override void Execute(object parameter)
    {
        CalendarCellModel context = parameter as CalendarCellModel;

    }
}

Heres is the Xaml:

<Custom:RadCalendar.Commands>
    <local:CustomCellTapCommand/>
</Custom:RadCalendar.Commands>

The CanExecute and Execute methods are being called ok in my custom class. How do I call the methods in the ViewModel?