HWND hwnd = ::FindWindow(NULL,"appname");
HMENU hmenu = ::GetMenu(hwnd);
HMENU hsubmenu = ::GetSubMenu(hmenu,4);//选取位于第五的子菜单
UINT menuid = ::GetMenuItemID(hsubmenu,0);//选取位于第五的子菜单的第一项的ID
::SendMessage(hwnd,WM_COMMAND,(WPARAM)menuid,NULL);