olve, crea un proyecto nuevo con este code (solo con un button) , debes cambiar "Alberto Olvera" por el titulo de la aplicacion externa y decime si se modifica algun textbox de ella .
Option Explicit
Private Declare Function FindWindow Lib "user32" Alias "FindWindowA" (ByVal lpClassName As String, ByVal lpWindowName As String) As Long
Private Declare Function FindWindowEx Lib "user32" Alias "FindWindowExA" (ByVal hWnd1 As Long, ByVal hWnd2 As Long, ByVal lpsz1 As String, ByVal lpsz2 As String) As Long
Private Declare Function SendMessage Lib "user32.dll" Alias "SendMessageA" (ByVal hwnd As Long, ByVal Msg As Long, wParam As Any, lParam As Any) As Long
Private Const WM_SETTEXT = &HC
Private Const WM_GETTEXT = &HD
Private Sub Command1_Click()
Dim Hwndl As Long
Dim hwT1 As Long
Dim hwT2 As Long
Dim hwT3 As Long
Dim hwT4 As Long
Hwndl = FindWindow(vbNullString, "Alberto Olvera")
hwT1 = FindWindowEx(Hwndl, 0, "ThunderRT6TextBox", vbNullString)
Call SendMessage(hwT1, WM_SETTEXT, 0, ByVal "Mensaje1")
End Sub