add menu item wordpress code example

Example 1: add top menu bar in wordpress

function register_my_menu() {
register_nav_menu('additional-menu',__( 'Additional Menu' ));
}
add_action( 'init', 'register_my_menu' );

Example 2: create menu in wordpress

function wpb_custom_new_menu() {
  register_nav_menus(
    array(
      'my-custom-menu' => __( 'My Custom Menu' ),
      'extra-menu' => __( 'Extra Menu' )
    )
  );
}
add_action( 'init', 'wpb_custom_new_menu' );
<?php
wp_nav_menu( array( 
    'theme_location' => 'my-custom-menu', 
    'container_class' => 'custom-menu-class' ) ); 
?>