FocusElement does not change focus when I set it using a DataTrigger - wpf

Question

I am making a window that shows the user multiple options, with each a radiobutton and a textbox.
When opening this window, the focus should be on the textbox corresponding to the radiobutton that is checked when opening the window (this is all preset, no need to worry about this).
It is important that it gets fixed either in xaml or in the code-behind.
I have used a DataTrigger to change the FocusedElement to the right textbox, however it gets overwritten after it is set.
The relevant code is in the DataTrigger below. The color gets changed correctly, however the FocusElement does not.
I have tried all the options that I have found on stackoverflow and google. The issue does not lie in the DataTrigger or the setting of the FocusedElement. I think it lies in the fact that it gets overridden at the end. I have used Snoop to see the changes in the Keyboard.FocusedElement and it does not show any change.
<DataTemplate x:Uid="DataTemplate_1" >
<RadioButton x:Uid="RadioButton_1" GroupName="Options" IsChecked="{Binding IsSelected}" Margin="4,0,0,0" Name="RadioBtn">
<StackPanel x:Uid="StackPanel_1" Orientation="Horizontal" >
<Label x:Uid="Label_1" Visibility="{Binding IsUserInput, Converter={cs:OppositeVisibilityConverter}}" Content="{Binding Description, Mode=OneWay}" />
<Label x:Uid="Label_2" Visibility="{Binding IsUserInput, Converter={cs:VisibilityConverter}}" Content="Anders, nl: " />
<TextBox x:Uid="MemAnders" Visibility="{Binding IsUserInput, Converter={cs:VisibilityConverter}}" Text="{Binding AlternativeText}"
Name="MemAnders" MinWidth="400" IsTabStop="False"
>
<TextBox.Style>
<Style TargetType="{x:Type TextBox}">
<Style.Triggers>
<DataTrigger Binding="{Binding IsSelected}" Value="True">
<Setter Property="FocusManager.FocusedElement" Value="{Binding ElementName=MemAnders}"/>
<Setter Property="Background" Value="LightGoldenrodYellow" />
</DataTrigger>
<DataTrigger Binding="{Binding IsSelected}" Value="False">
<Setter Property="FocusManager.FocusedElement" Value="{Binding ElementName=RadioBtn}"/>
<Setter Property="Background" Value="LightBlue" />
</DataTrigger>
</Style.Triggers>
</Style>
</TextBox.Style>
</TextBox>
</StackPanel >
</RadioButton >
</DataTemplate >
The textbox corresponding to the checked radiobutton should be focused. Instead another (parent?) object is focused.
Anyone know a workaround for this?

Answer 1

The work-around turned out to be setting focus to the textbox in the Window_Loaded function.
I used an algorithm provided by Find a WPF element inside DataTemplate in the code-behind to find the correct textbox element. Then I did TextBox.Focus(); and that fixed it.