<VerticalStackLayout>
        <telerik:RadCheckBox x:Name="checkbox">
            <telerik:RadCheckBox.Behaviors>
                <telerik:RadEventToCommandBehavior EventName="IsCheckedChanged" Command="{Binding CheckedCommand}" />
            </telerik:RadCheckBox.Behaviors>
        </telerik:RadCheckBox>
    </VerticalStackLayout>

public class ViewModel : NotifyPropertyChangedBase
{
    public ViewModel()
    {
        this.CheckedCommand = new Command<IsCheckedChangedEventArgs>(this.CheckBoxUserCommand);
    }

    private void CheckBoxUserCommand(IsCheckedChangedEventArgs obj)
    {
        // logic here:
    }

    public ICommand CheckedCommand { get; set; }  
}