Hi,
There is no option to do that.
You'll have to edit the file "after_end" of the view "checkout" via the menu Display>Views to add custom code for that.
If you're a developer, it won't be a problem, although it's not just a few lines of code. You'll have to first get the order_id from the user state, and then load the order data:
www.hikashop.com/forum/4-how-to/68733-ge...id-in-after-end.html
Then, you can look at $order->products to identify the products of the order and add your share button custom HTML/JS/PHP based on that.