C / C++ / MFC

void CMyWnd::OnKeyUp(UINT nChar, UINT nRepCnt, UINT nFlags)
{
        CWnd::OnKeyUp(nChar, nRepCnt, nFlags);
        ::SendMessage(m_hWndTarget, WM_KEYUP, (WPARAM) nChar, (LPARAM) ((nFlags * 256) + 1))
}

LRESULT CMyWnd::DefWindowProc(UINT message, WPARAM wParam, LPARAM lParam) 
{
	// Pass key presses to other window
	switch(message)	
	{
	case WM_KEYUP:
	case WM_KEYDOWN:
	case WM_SYSKEYUP:
	case WM_SYSKEYDOWN:
		::SendMessage(m_hWndTarget, message, wParam, lParam);
		break;
	}
	return CWnd::DefWindowProc(message, wParam, lParam);
}