Another custom menu to insert in the header

Hello everybody
i need to add a custom menu exactly above main menu in the header (I have already activated the secondary menu, it is located at the top of the page).
I have modified the function.php file in my child theme and now i would like to add my new menu in the header.
I suppose the file in which insert the line
<?php wp_nav_menu( array( 'theme_location' => 'header-menu' ) ); ?>
is helper-main-menu.php but i don’t know where to insert it.

Could you help me, please.
Thank you in advance.

Andrea