I would like to add an option (select) to a menu. I am using hook_form_menu_edit_form_alter to add a select element and an additional submit function. The select is shown and my submit function is called.
I don't know how to save the value and how to retrieve it. In my submit function I can get the menu entity by
$menu = $form_state->getFormObject()->getEntity();
But I can't find anything similar to options like for the single menu links.