How do I add a custom sub menu menu under Woo-commerce marketing?

If available the $GLOBALS[ 'menu' ] array holds data for each top level menu item in the dashboard, and in the sub array for the Woocommerce “Marketing” menu item, key 2 has the value you’re looking for: "woocommerce-marketing".

So with that your add_submenu_page should look like the below to make your sub menu item show up under the Woocommerce Marketing menu item:

add_submenu_page(
    'woocommerce-marketing',
     __( 'Sub Menu' ),
    'manage_woocommerce',
    'sub-menu-slug',
    'sub_menu_page_callback'
);