To achieve this functionality in the MVVM scenario you will need to implement it with custom code. For example you can subscribe for the RadTreeView's KeyDown event and inside of it get the currently selected note and check it. The following code snippet demonstrates this approach for the MVVM implementation from the related help article
sender, KeyEventArgs e)
(Keyboard.Modifiers == ModifierKeys.Control && e.Key == Key.Space)
var selectedCategory = (CategoryViewModel)
selectedCategory.IsChecked = !selectedCategory.IsChecked;
Please try this and let me know if it works for you.
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