Add a custom link to the end of a menu that uses the wp_nav_menu() function

append-nav-items.php

<?php

/**
* Add a custom link to the end of a specific menu that uses the wp_nav_menu() function
*/
add_filter('wp_nav_menu_items', 'add_admin_link', 10, 2);
function add_admin_link($items, $args){
    if( $args->theme_location == 'footer_menu' ){
        $items .= '<li><a title="Admin" href="'. esc_url( admin_url() ) .'">' . __( 'Admin' ) . '</a></li>';
    }
    return $items;
}