How to get access to m_hwnd from CTaskDialog

Question

I derive a class from CTaskDialog to implement an AutoCloseButton.

 HRESULT CMyTaskDialog::OnTimer(_In_ long lTime)
 {
    if (GetTickCount() - m_dwStart > (DWORD)m_nTimeout)
    {
        SendMessage(m_hWnd, TDM_CLICK_BUTTON, IDOK, 0);
    }
    
    return CTaskDialog::OnTimer(lTime);
}

Unfortunately m_hWnd is a private member in CTaskDialog, so I have no accces to it.

The CTaskDialog has the member

friend HRESULT CALLBACK TaskDialogCallback(_In_ HWND hwnd, _In_ UINT uNotification, _In_ WPARAM wParam, _In_ LPARAM lParam, _In_ LONG_PTR dwRefData);

But it is not possible to derive it (LNK2005 error (already defined in afxtaskdialog.obj) or I don't know how to do this.