Set Focus on a Sub Widget TextBox in XAML
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How can I set the default focus to be on a TextBox which is a sub widget in my current view?

    <StackPanel Orientation="Vertical" FocusManager.FocusedElement="{Binding ElementName=SomeWidget.UnderTextBoxName}">
       <MyWidget x:Name="SomeWidget" Text="A" />
       <TextBox x:Name="TxtB" Text="B" />
    </StackPanel>

...thought that might work, but no luck?