If we know menu slug then we can use that. use like following (change MENU_SLUG with your menu slug)
add_filter( 'wp_nav_menu_MENU_SLUG_items', function( $items, $args ) {
$items .= '<li><a href="'. fep_query_url( 'messagebox' ) .'">' . sprintf(__('Message Box%s', 'front-end-pm'), fep_get_new_message_button() ) . '</a></li>';
return $items;
}, 99, 2);