marketing-hub-add-submenu.php

<?php
/**
 * Plugin Name: Add submenu item to WooCommerce Marketing Hub
 */

add_action('admin_menu', 'register_my_custom_submenu_page');

function register_my_custom_submenu_page() {

	add_submenu_page(
		'woocommerce-marketing',
		'Coupon Generator',
		'Coupon Generator',
		'manage_woocommerce',
		'coupon_generator',
		function(){ echo('test'); }
	);
}

add_filter( 'woocommerce_marketing_menu_items', function( $items ) {
	$items[] = array(
		'id'    => 'woocommerce-marketing-test',
		'title' => __( 'Test', 'woocommerce-admin' ),
		'path'  => '/marketing/test',
	);
	return $items;
} );