login and logout links in the header

Hi,

I want to use the tecnique described here: https://www.datafeedr.com/dynamically-add-link-to-wordpress-menu-based-on-users-logged-in-status/ to add the login and logout links in the header

/**
* Adds a link to the Primary Menu based on the
* user's logged in status. This is to be placed
* in your theme's functions.php file or in your
* own plugin file.
*
* This example works for the WordPress Twenty-* Twelve theme.
*/

function twentytwelve_wp_nav_menu_items($items, $args) {

// Make sure this is the Primary Menu.
// You may need to modify this condition
// depending on your theme.

if ($args->theme_location == 'primary') {

// CSS class to use for <li> item.

	$class = 'menu-item';

	if (is_user_logged_in()) {

// User is logged in, link to welcome page.

		$extra = '<li id="menu-item-logged-in-user" class="'.$class.'"><a href="/welcome">'.__('Welcome').', '.wp_get_current_user()->user_login.'!</a></li>';
	} 	

	else {

// User is guest, link to login page.

		$extra = '<li id="menu-item-logged-out-user" class="'.$class.'"><a href="/wp-login.php">'.__('Log in').'</a></li>';
	}

// Add extra link to existing menu.
	$items = $items . $extra;
}

// Return menu items.

return $items;

}

// Hook into wp_nav_menu_items.

add_filter( 'wp_nav_menu_items', 'twentytwelve_wp_nav_menu_items', 10, 2 );

to me the main problem it’s here ($args->theme_location == 'primary')

I tried to use “Main Menu” as theme_location but it didn’t worked.

Do you have any suggestion?