Hi Akvarius 

At the moment, the cart does not present actual separate pages during the checkout process.

For this reason, what you need to achieve is currently not possible since those pages are simply different layouts for the default index page.

You could try and insert the code in a different page if you're using the PayPal payment method, since it allows for automatic redirect to a given success page.

If this was not okay for you, it is currently impossible for the code to be inserted only on a specific step of the purchase process, if not with external custom code for which the Staff cannot unfortunately offer support

Let me know if I can provide more information on this

Thank you

Stefano