lol.php

<?php

use WHMCS\View\Menu\Item as MenuItem;

add_hook(
    'ClientAreaPrimarySidebar', 
    1, 
    function(MenuItem $primarySidebar)
    {
        if ($primarySidebar->hasChild('My Account')) {
            $primarySidebar->getChild('My Account')
                ->addChild(
                    'Mailing List Subscription Prefs', 
                    array(
                        'label' => 'Subscription Preferences',
                        'uri' => 'subscriptionprefs.php',
                        'order' => '100',
                        'attributes' => array(
                            'target' => '_blank',
                        ),
                    )
                );
        }
    }
);