public MainWindow()
{
    InitializeComponent();
    this.radSpreadsheet.ActiveSheetChanged += RadSpreadsheet_ActiveSheetChanged;
    
    this.Loaded += MainWindow_Loaded;
}

private void MainWindow_Loaded(object sender, RoutedEventArgs e)
{
    RegisterFillRight();
}

private void RadSpreadsheet_ActiveSheetChanged(object sender, EventArgs e)
{
    RegisterFillRight();
}
public void RegisterFillRight()
{
    RadWorksheetEditor worksheetEditor = this.radSpreadsheet.ActiveWorksheetEditor;

    if (this.radSpreadsheet.ActiveWorksheetEditor != null)
    {
        worksheetEditor.KeyBindings.RegisterCommand(worksheetEditor.Commands.FillRight, Key.R, ModifierKeys.Control);
    }
}