如果要在每次页面调用时动态添加项目，可以查看以下答案：
Dynamically add sub-categories to any category in the menu

要直接添加元素，您可以尝试将其用于自定义链接，例如：

// create the new element

$post = [
    \'post_title\' => "StackExchange " . date("H:i:s"),
    "menu_order" => 700, // position in the menu
    \'post_type\' => \'nav_menu_item\',
    \'post_status\' => \'publish\',
];

$menu_item_db_id = wp_insert_post( $post );

update_post_meta( $menu_item_db_id, \'url\', "https://wordpress.stackexchange.com/");
update_post_meta( $menu_item_db_id, \'type\', "custom");
update_post_meta( $menu_item_db_id, \'_menu_item_menu_item_parent\', 0); // change to create a sub menu


// association to the menu

$menuLocations = get_nav_menu_locations();
$menuID = (int) $menuLocations[\'primary\'];

wp_set_object_terms($menu_item_db_id, $menuID, "nav_menu");