Get selected nodes from treeview UWP

I am developing UWP app, and I am struggling with treeview checkboxes. I need to know which nodes have checkboxes checked.

I have tried using

mytreeview.SelectedNodes 

but it always returns null. This is my treeview I have also tried using

<TreeViewItem IsSelected="{x:Bind IsSelected ,Mode=TwoWay}" ...>

and in page.cs

public bool IsSelected { get; set; } = false;

but when I check the checkbox it is still false