WordPress programmatically filter menu items given a location

gistfile1.php

/**
 * Filter Menu Items
 */
function tdp_filter_menu_items( $items, $args ) {
    
    if($args->theme_location == 'primary_menu')
    {
        
    		$items .= '<li class="primary-menu-secondary-item"><a href="#">Menu Item</a></li>';
    }

    return $items;
}
add_filter( 'wp_nav_menu_items', 'tdp_filter_menu_items', 10, 2);