App.xaml

<Application x:Class="FocusSample.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             StartupUri="MainWindow.xaml">
    <Application.Resources>
         
    </Application.Resources>
</Application>

App.xaml.cs

using System;
using System.Collections.Generic;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Threading.Tasks;
using System.Windows;

namespace FocusSample
{
    /// <summary>
    /// App.xaml の相互作用ロジック
    /// </summary>
    public partial class App : Application
    {
    }
}

MainWindow.xaml

<Window x:Class="FocusSample.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:c="clr-namespace:FocusSample"
        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}"/>
                            <Setter Property="Width" Value="100"/>
                        </DataTrigger>
                    </Style.Triggers>
                </Style>
            </Button.Style>
            toplevelのtextboxへfocus
        </Button>
        <TextBox x:Name="DummyInput" Text="Dummy"></TextBox>
        <c:WrappedTextBox x:Name="WrappedInput"/>
        <Button Height="50" x:Name="Button2">
            UserControlのtextboxへfocus
            <Button.Style>
                <Style TargetType="Button">
                    <Style.Triggers>
                        <DataTrigger Binding="{Binding  ElementName=Button2, Path=IsMouseOver}" Value="True">
                            <Setter Property="FocusManager.FocusedElement"
                                    Value="{Binding ElementName=WrappedInput}"/>
                            <Setter Property="Width" Value="100"/>
                        </DataTrigger>
                    </Style.Triggers>
                </Style>
            </Button.Style>
        </Button>
    </StackPanel>
</Window>

MainWindow.xaml.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace FocusSample
{
    /// <summary>
    /// MainWindow.xaml の相互作用ロジック
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }
    }
}

WrappedTextBox.xaml

<UserControl x:Class="FocusSample.WrappedTextBox"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             mc:Ignorable="d" 
             FocusManager.IsFocusScope="True"
             GotFocus="Container_GotFocus"
             x:Name="Container"
             d:DesignHeight="300" d:DesignWidth="300">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition/>
        </Grid.RowDefinitions>
        <TextBox Grid.Row="0" x:Name="A" Background="AliceBlue"></TextBox>
    </Grid>
</UserControl>

WrappedTextBox.xaml.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace FocusSample
{
    /// <summary>
    /// WrappedTextBox.xaml の相互作用ロジック
    /// </summary>
    public partial class WrappedTextBox : UserControl
    {
        public WrappedTextBox()
        {
            InitializeComponent();
        }

        private void Container_GotFocus(object sender, RoutedEventArgs e)
        {
            var input = this.A;
            FocusManager.SetFocusedElement(FocusManager.GetFocusScope(sender as DependencyObject), input);
            e.Handled = true;
        }
    }
}