open_folder(dname)

// implementation
function open_folder(dname) {
  var escapedfolder = string_replace_all(dname, "\"", "\\\"");
  if (os_type = os_windows) execute_shell("cmd", "explorer \"" + escapedfolder + "\"");
  else if (os_type = os_macosx) execute_shell("open", "\"" + escapedfolder + "\"");
  else if (os_type = os_linux) execute_shell("xdg-open", "\"" + escapedfolder + "\"");
}

// usage example
open_folder(working_directory);