Hi,
You can do it with the HikaShop coupon code, by creating discount to have a free product.
Or you can do such thing using HikaSerial.
With HikaSerial you can use the "consume" feature and let the user enter a serial to perform a special action.
It can be adding a special product in the cart.
It is also possible to create a custom plugin in order to perform some specific action, like let the user access to a special download area, launch a special download, etc. (but it has to be developed).
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.