php generated custom menu item

I modified my theme navigation by adding a custom menu item like this

<nav>
        <?php wp_nav_menu(array(
                                'theme_location' => 'top-menu',
                                'container' => '',
                                'fallback_cb' => 'show_top_menu',
                                'echo' => true,
                                'walker' => new description_walker(),
                                'depth' => 1,
								'items_wrap' => '<ul>%3$s<li id="item-id"> <strong>CUSTOM MENU ITEM</strong> </li></ul>'
								) ); ?>
    </nav>

I would like to generate this custom menu item using a shorctode…

<?php echo do_shortcode(<strong>MY SHORCODE</strong>) ?>

How can i replace my CUSTOM MENU ITEM with MY SHORCODE ?

i know basic html and css but i am total newbie when it comes to php. Any help would be appreciated.