Source code for https://victorfont.com/add-class-wordpress-menu-ul-element/

add_class_to_wp_menu_ul.php

<?php

add_filter( 'wp_nav_menu_args', 'my_custom_ul_class' );
function my_custom_ul_class( $args ) {
  $args['menu_class'] .= ' my-custom-class'; return $args;
}