Public Class Form1
    Declare Auto Function GetWindow Lib "user32.dll" (ByVal hWnd As IntPtr, _
      ByVal uCmd As Int32) As IntPtr
    Declare Auto Function SendMessage Lib "user32" (ByVal hwnd As IntPtr, ByVal _
    wMsg As Int32, _
            ByVal wParam As Int32, ByVal lParam As String) As Int32
    Const WM_SETTEXT As Int32 = &HC
    Const GW_CHILD As Int32 = &H5
    Dim p As Process = Process.Start("notepad")
 
    Private Sub TextBox1_TextChanged(ByVal sender As Object, ByVal e As _
      System.EventArgs) Handles TextBox1.TextChanged
        SendMessage(GetWindow(p.MainWindowHandle, GW_CHILD), WM_SETTEXT, 0, _
        TextBox1.Text)
    End Sub
End Class