I have the following, which successfully inserts a dynamic link as the last top-level menu item of the specified menu:

add_filter( 'wp_nav_menu_items', 'example_last_nav_item', 10, 2 );

function example_last_nav_item( $items, $args ) {
    if ( $args - > theme_location == 'account-menu' && is_user_logged_in() ) {
        $current_user = wp_get_current_user();
        $items .= '<li class="menu-item profile-link"><a href="/' . strtolower( str_replace( ' ', '-', $current_user - > user_login ) ) . '/">Profile</a></li>';
    }
    return $items;
}

But, how would I access this menu's sub-menu and insert the link as the first item of the sub-menu?

Account Menu Top-level

— DYNAMIC LINK HERE

— Sub-menu Item #2

— Sub-menu Item #3

I have the following, which successfully inserts a dynamic link as the last top-level menu item of the specified menu:

add_filter( 'wp_nav_menu_items', 'example_last_nav_item', 10, 2 );

function example_last_nav_item( $items, $args ) {
    if ( $args - > theme_location == 'account-menu' && is_user_logged_in() ) {
        $current_user = wp_get_current_user();
        $items .= '<li class="menu-item profile-link"><a href="https://example/profile/' . strtolower( str_replace( ' ', '-', $current_user - > user_login ) ) . '/">Profile</a></li>';
    }
    return $items;
}

But, how would I access this menu's sub-menu and insert the link as the first item of the sub-menu?

Account Menu Top-level

— DYNAMIC LINK HERE

— Sub-menu Item #2

— Sub-menu Item #3

• menus

2 Answers 2

The following code will go through all items and search the first item that has a parent. If found, new item with the same parent will be inserted as "first child".

add_filter( 'wp_nav_menu_objects', 'se336857_dynamic_submenuitem', 20, 2 );
function se336857_dynamic_submenuitem( $items, $args )
{
    if ( !is_user_logged_in() || $args->theme_location != 'account-menu' )
        return $items;

    $parent = false;
    $current_user = wp_get_current_user();
    $url = '/profile/' . strtolower(str_replace( ' ', '-', $current_user->user_login ));

    $item_profile = [
        'title' => __('Profile'),
        'url'   => home_url( $url ),
        'classes' => 'menu-item profile-link',
    ];
    $item_profile = (object)$item_profile;
    $item_profile = new WP_Post($item_profile);
    $i = -1;
    foreach ( $items as $item )
    {
        ++$i;
        if ( $item->menu_item_parent == 0 )
            continue;
        //
        // first submenu item
        $item_profile->menu_item_parent = $item->menu_item_parent;
        $new_items = [ $item_profile ];
        array_splice($items, $i, 0, $new_items );
        break;
    }
    return $items;
}

I recommend you to use wp_nav_menu_objects hook, which allows you to work with menu items as objects, instead of a string. Inside of your if statement you can create as many items as you want.

add_filter( 'wp_nav_menu_objects', 'example_last_nav_item', 10, 2 );

function example_last_nav_item( $items, $args ){

    if ( $args->theme_location=='account-menu' && is_user_logged_in() ){

        //parent menu item
        $parent_item = array(
            'title'            => __('Profile'),
            'menu_item_parent' => 0, //PARENT ELEMENT HAS 0
            'ID'               => 'profile',
            'db_id'            => 9999, //MAKE SURE IT's UNIQUE ID
            'url'              => 'http://google',
            'classes'          => array( 'custom-menu-item' )
        );

        //submenu menu item
        $submenu_item = array(
            'title'            => __('Submenu first element'),
            'menu_item_parent' => 9999, //FOR SUBMENU ITEM SET PARENT's db_id
            'ID'               => 'submenu-item',
            'db_id'            => 99992,
            'url'              => 'http://google',
            'classes'          => array( 'custom-menu-item' )
        );

        $items[] = (object) $parent_item;
        $items[] = (object) $submenu_item;
    }

    return $items;

}

About all item properties you can read here