您可以这样做：

主窗口.xaml

<Grid>
    <Grid.Resources>
        <CommandBarFlyout
            x:Name="TestCommanBarFlyout"
            Placement="Right">
            <AppBarButton Icon="Copy" />
            <AppBarButton Icon="Delete" />
        </CommandBarFlyout>
    </Grid.Resources>
    <TreeView ItemsSource="{x:Bind Items}">
        <TreeView.ItemTemplate>
            <DataTemplate x:DataType="x:String">
                <TreeViewItem
                    Content="{x:Bind}"
                    ContextRequested="TreeViewItem_ContextRequested" />
            </DataTemplate>
        </TreeView.ItemTemplate>
    </TreeView>
</Grid>

主窗口.xaml.cs

public ObservableCollection<string> Items { get; } = new() { "A", "B", "C" };

private void TreeViewItem_ContextRequested(UIElement sender, ContextRequestedEventArgs args)
{
    if (sender is TreeViewItem item)
    {
        item.IsSelected = true;
        TestCommanBarFlyout.ShowAt(
            item,
            new FlyoutShowOptions()
            {
                ShowMode = FlyoutShowMode.Standard
            });
    }
}