Hi,
You're the first one bringing this out.
By default, the system will redirect back to the product details page.
This is done with the line:
$app->redirect(hikashop_contentLink('product&task=show&cid='.$product->product_id.'&name='.$product->alias.$url_itemid, $product, false, true));
in the file components/com_hikashop/controllers/product.php
So a solution, if I understand correctly your problematic, would be to add an extra parameter to that redirect line of code.
For example:
$app->redirect(hikashop_contentLink('product&task=show&contact_request_sent=1&cid='.$product->product_id.'&name='.$product->alias.$url_itemid, $product, false, true));
So that you would have the extra parameter "contact_request_sent" in the URL of the product page. Would that work for your need ?