focus

gistfile1.xml

<Window x:Class="FocusSample.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Window.Resources>
        <Style TargetType="TextBox">
            <Setter Property="Height" Value="50"/>
            <Setter Property="FontSize" Value="40"/>
        </Style>
    </Window.Resources>
    <StackPanel>
        <TextBox x:Name="Input"></TextBox>
        <Button Height="50" x:Name="Button">
            <Button.Style>
                <Style TargetType="Button">
                    <Style.Triggers>
                        <DataTrigger Binding="{Binding  ElementName=Button, Path=IsMouseOver}" Value="True">
                            <Setter Property="FocusManager.FocusedElement"
                                    Value="{Binding ElementName=Input}"/>
                        </DataTrigger>
                    </Style.Triggers>
                </Style>
            </Button.Style>
        </Button>
        <TextBox x:Name="DummyInput"></TextBox>
    </StackPanel>
</Window>