Add an item to the top-menu

I’d like to add an additional item/field (just a text with a hyperlink) at the top of my blog, in the menu-top, where pages links are located.

I added a new filter to my theme functions.php file:

add_filter('wp_nav_menu_nav', 'add_admin_link', 10, 2);

function add_admin_link($items, $args){
    if( $args->theme_location == 'primary' ){
        $items .= '<li><a title="Admin" href="'. esc_url( admin_url() ) .'">' . __( 'Admin' ) . '</a></li>';
    }
    return $items;
}

I’m supposing it must be the “primary” location as this is a value passed to this menu from my header.php:

<?php wp_nav_menu( array( 'theme_location' => 'primary', ...

But it won’t display text and link.

If remove the if( $args->theme_location == 'primary' ) condition – the text and link are displayed but on the right-column instead of top of the page.

What I’m doing wrong here?

P.S. I’m not a PHP-developer.

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