Add class to specific WordPress navigation menu link

gistfile1.txt

/**
 * Add class to specific WordPress navigation menu link
 * Reference: https://codex.wordpress.org/Plugin_API/Filter_Reference/nav_menu_link_attributes
 */
add_filter('nav_menu_link_attributes', 'ivycat_account_secondary_menu_link_attributes', 10, 4);

function ivycat_account_secondary_menu_link_attributes($atts, $item, $args, $depth){
    if ($args->menu->slug == 'utility' && $item->ID == 26){

        $class = "account_button";

        // Make sure not to overwrite any existing classes
        $atts['class'] = (!empty($atts['class'])) ? $atts['class'].' '.$class : $class; 
    }
    return $atts;
}

Notes:
In this example:

1. utility is the slug for the custom menu
2. 26 is the ID of that specific menu item
3. account_button is the class or classes that you want to add to the link.