Add “onclick” option to header menu item

I went to the GeneratePress site and looked at one of their demos:

<nav id="site-navigation" class="main-navigation sub-menu-left" itemtype="https://schema.org/SiteNavigationElement" itemscope>
    <div class="inside-navigation">
        <button class="menu-toggle" aria-controls="primary-menu" aria-expanded="false">
            <span class="mobile-menu">Menu</span>
        </button>
        <div id="primary-menu" class="main-nav">
            <ul id="menu-menu-1" class=" menu sf-menu">

                <li id="menu-item-594520" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-home menu-item-594520"><a href="https://generatepress.com/">area riservata</a></li>

                <li id="menu-item-594521" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-594521"><a href="https://generatepress.com/premium/">Premium</a></li>

            </ul>
        </div>
    </div><!-- .inside-navigation -->
</nav><!-- #site-navigation -->

So in your function’s PHP you can try dropping this:

function change_area_riservata_url() { ?>
    <script type="text/javascript">
        jQuery( document ).ready( function($) {
            <?php if( is_user_logged_in() ) : ?>
                var hrefAtt="https://google.com"; //the URL if user is logged in
            <?php else : ?>
                var hrefAtt="https://bing.com"; //the URL if NOT logged in
            <?php endif; ?>
            //Make sure the correct ID is in place
            $( '#menu-item-594520 a' ).attr( 'href', hrefAtt );
        } );
    </script>
} ?>
add_action( 'generate_after_header', 'change_area_riservata_url', 20 );

Obviously you’ll want to edit the URLs and the ID of the menu item so that it matches your site.