Code:
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 Const WM_LBUTTONDOWN = &H201
Private Const WM_LBUTTONUP = &H202
Private Const MK_LBUTTON = &H1

Private Sub Command1_Click()
    'Send a click to a window
    PostMessage HwndOfWindow, WM_LBUTTONDOWN, MK_LBUTTON, 0
    PostMessage HwndOfWindow, WM_LBUTTONUP, MK_LBUTTON, 0
End Sub