Private Declare Function FindWindow Lib "user32" _
  Alias "FindWindowA" (ByVal sClassName As String, _
  ByVal lpWindowName As String) As Long
 
Private Declare Function PostMessage Lib "user32" Alias "PostMessageA" ( _
    ByVal hwnd As Long, ByVal wMsg As Long, _
    ByVal wParam As Long, ByVal lParam As Long) As Long

Private Sub Command1_Click()
Dim FensterTitel As String
    Dim lHWND As Long
    FensterTitel = "FENSTER TITEL" ' genauer Fenstertitel
    lHWND = FindWindow(vbNullString, FensterTitel)
    Call PostMessage(lHWND, WM_CLOSE, 0, 0&)
End Sub