Private Declare Function SendMessage Lib "user32" Alias "SendMessageA" (ByVal hwnd As Long, ByVal wMsg As Long, ByVal wParam As Long, ByVal lParam As Any) As Long
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 hWndParent As Long, ByVal hWndChildAfter As Long, ByVal lpClassName As String, ByVal lpWindowName As String)

Private Const WM_SETTEXT = &HC

Private Sub Command1_Click()
Dim lhWnd As Long
Dim Fhwnd As Long
Fhwnd = FindWindow(vbNullString, "Form1")
lhWnd = FindWindow(Fhwnd, Text1.hwnd)
SendMessage lhWnd, WM_SETTEXT,  0 , "Новый текст"
End Sub

Dim lhWnd As Long
Dim Fhwnd As Long
Dim Fhwnd2 As Long
Fhwnd = FindWindow(vbNullString, "Выбор файла")
Fhwnd2 = FindWindowEx(Fhwnd,  0 , "ComboBoxEx32", vbNullString)
SendMessage Fhwnd2, WM_SETTEXT,  0 , ByVal "Текст"

lhWnd = FindWindowEx(Fhwnd,  0 , "Button", "Открыть")
SendMessage lhWnd, BM_CLICK,  0 ,  0