It depends on your scenario. You can use SelectedItem
property of the TreeView to get the item which is selected and then you can work with your business collection and add items in the view models. If you are using ObservableCollection the treeView will be automatically updated when you update its ItemsSource.
I hope this information helps. Please let us know if you need any further assistance. In this case could you elaborate a bit more on your scenario by sending us a sample project or code-snippet. E.g. it would be useful to see the structure of your view models and if you need to add the node under the selected item on the same level or as its child. Thank you in advance for your cooperation.
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