Cannot create a sub-menu under woocommerce

I was trying to add a sub menu under the root menu ‘WooCommerce’. I set the root menu as followings,
$this->setRootMenuPageBySlug( ‘edit.php?post_type=shop_order’ );

then add the sub menu,
$this->addSubMenuItems(
array(
‘title’ => ‘My Title’, // the page and menu title
‘page_slug’ => ‘my_slug’ // the page slug
)
);

However it’s not working. I just cannot see ‘My Title’ under WooCommerce.

PS. It can be working if I put it under WooCommerce’s ‘Product’ menu if I set the root to be,
$this->setRootMenuPageBySlug( ‘edit.php?post_type=product’ );

Kindly advise. Thank you.