Add custom attribute to specific WordPress menu item

/*
 * Add custom lightbox attribute to the menu
*/ 
function prefix_menu_item_extra_atts( $atts, $item, $args )
{
	if ( $item->object_id == '977') {
		 $atts['data-optin-slug'] = 'qykfszblau-lightbox';
		 $atts['class'] = 'manual-optin-trigger';
	}
   
    return $atts;
}
add_filter( 'nav_menu_link_attributes', 'prefix_menu_item_extra_atts', 10, 3 );

Where do we put this code?