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