Set the focus on a textbox in xaml wpf

You can apply this property directly on the TextBox :

<TextBox Text="{Binding MyText}" FocusManager.FocusedElement="{Binding RelativeSource={RelativeSource Self}}"/>

You can use the FocusManager.FocusedElement attached property for this purpose. Here's a piece of code that set the focus to TxtB by default.

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

You can also use TxtB.Focus() in your code-behind if you don't want to do this in XAML.