Question

I am working on a plugin, which creates a couple of Virtual pages, and I wish these links to be available in Menu admin page, to let users have the liberty to add them as they create menus.

I want to add a Meta box in Menu administration, very similar to Page/Category meta boxes, to let users select what page to add in their menu.

Answer 1

Apparently, the only possible research is in the core itself.

Here, /wp-includes/nav-menu.php, we can get how to insert the meta box:

add_action('admin_init', 'so_13875144_nav_menu_meta_box');

function so_13875144_nav_menu_meta_box() {
    add_meta_box( 
        'my-custom-nav-box', 
        __('Custom Box'), 
        'so_13875144_display_menu_custom_box', 
        'nav-menus', 
        'side', 
        'default' 
    );
}

function so_13875144_display_menu_custom_box() {
    /* Not sure about this global var */
    //global $_nav_menu_placeholder;
    //$_nav_menu_placeholder = ( 0 > $_nav_menu_placeholder ) ? intval($_nav_menu_placeholder) - 1 : -1;
    ?>
        <p id="menu-item-custom-box">
            <label class="howto" for="custom-menu-item-custom-box">
                <span><?php _e('URL'); ?></span>
                <input id="custom-menu-item-custom-box" name="menu-item[<?php echo $_nav_menu_placeholder; ?>][menu-item-custom-box]" type="text" class="code menu-item-textbox" value="my text" />
            </label>
        </p>
    <?php
}

But, the hard part, which I haven’t managed to make work, is to save the value.
This is the file /wp-admin/nav-menus.php that has to be studied.

Tried to hook into the action wp_update_nav_menu, but the custom meta box input field is not being passed into $_POST.

WordPress Answers may have some hint: https://wordpress.stackexchange.com/search?q=wp_update_nav_menu