Wpf textbox inside WPF contextmenu item opened in winform is not working

<UserControl x:Class="SearchTextBoxMenu"
             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" Margin="-34,0,-49,0">
    <UserControl.Resources>
        <BooleanToVisibilityConverter x:Key="B2VC"/>
    </UserControl.Resources>
    <MenuItem>
        <MenuItem.Template>
            <ControlTemplate>
                <Grid removed="White">
                    <TextBlock Name="placeHolderText" Text="Escriba aquí.." Margin="5,2,20,2" Foreground="Gray" 
                               Visibility="{Binding ElementName=textBox,Path=Text.IsEmpty,Converter={StaticResource B2VC}}"/>
                    <TextBox  Name="textBox" BorderBrush="Transparent" removed="Transparent" Width="150" Padding="3,0,20,0" 
                              PreviewGotKeyboardFocus="_keyboardFocusChangedEventArgs" />
                    <Button Name="searchButton" Width="20" HorizontalAlignment="Right" BorderBrush="Transparent" Background="Transparent">
                        <Canvas >
                            <Path StrokeThickness="1" Stroke="Black" Height="10.167" Stretch="Fill" Width="10.167" Canvas.Left="-6.875" Canvas.Top="-6.875">
                                <Path.Data>
                                    <EllipseGeometry Center="-2,-2" RadiusX="5" RadiusY="5" />
                                </Path.Data>
                            </Path>
                            <Path StrokeThickness="1" Stroke="Black" StrokeEndLineCap="Round" Height="5.291" Stretch="Fill" Width="5.073" Canvas.Left="1.104" Canvas.Top="1">
                                <Path.Data>
                                    <LineGeometry StartPoint="1.5,1.5" EndPoint="5.5,5.5" />
                                </Path.Data>
                            </Path>
                        </Canvas>

                    </Button>
                </Grid>
            </ControlTemplate>
        </MenuItem.Template>
    </MenuItem>
</UserControl>

Imports System.Windows
Imports System.Windows.Media

Public Class SearchTextBoxMenu
    'Avoid losing focus when mouse leaves
    Private Sub _keyboardFocusChangedEventArgs(sender As Object, e As Windows.Input.KeyboardFocusChangedEventArgs)
        Dim pm As UIElement = FindParent(Of Controls.ContextMenu)(sender)
        If pm IsNot Nothing AndAlso pm.IsKeyboardFocusWithin Then
            e.Handled = False
        End If
    End Sub

    Private Function FindParent(Of T As DependencyObject)(child As DependencyObject) As T
        Dim parentObject As DependencyObject = VisualTreeHelper.GetParent(child)
        If parentObject Is Nothing Then Return Nothing
        Dim parent As T = TryCast(parentObject, T)

        If parent IsNot Nothing Then
            Return parent
        Else
            Return FindParent(Of T)(parentObject)
        End If

    End Function

End Class

Imports System.Windows

Public Class Form1

    Dim cMenu As New Controls.ContextMenu

    Sub New()

        InitializeComponent()
        Dim txt As New SearchTextBoxMenu
        cMenu.Items.Add(txt)
        txt.IsManipulationEnabled = True

    End Sub


    Private Sub Form1_MouseClick(sender As Object, e As MouseEventArgs) Handles Label1.MouseClick
        cMenu.Placement = Windows.Controls.Primitives.PlacementMode.MousePoint
        cMenu.IsOpen = True
    End Sub
End Class

MyWpfProject.MainWindow mw = new MyWpfProject.MainWindow();
ElementHost.EnableModelessKeyboardInterop(mw);
mw.Show();