Menu callback: Provides the menu link submission form.
\Drupal\system\Plugin\Core\Entity\Menu $menu: An entity representing a custom menu.
Returns the menu link submission form.
function menu_link_add(Menu $menu) {
$menu_link = entity_create('menu_link', array(
'mlid' => 0,
'plid' => 0,
'menu_name' => $menu
->id(),
));
drupal_set_title(t('Add menu link'));
return entity_get_form($menu_link);
}