There’s a thread on this somewhere. You have to make a donation to the page and then you’ll get given a code. I’m sure someone will point you in the right direction.
You should only need the code if you transfer the radio to another vehicle. It's intended to deter thieves, as a stolen radio won't work in a different vehicle without the code.
A copy of the code is generally provided with the vehicle documentation (often on a small card); it's something that should be checked is there if purchasing a secondhand car (and possibly even a new one).
On some marques of car (but generally not Fiat), you might also need the code if the radio loses its permanent live feed, for example if the battery were removed or went completely flat. Fiat store a copy of the code in the car's body computer, and it automatically retrieves it from there if power to the radio is interrupted.
However, the deterrent value of the system is essentially broken, since it's not hard to find the code for most car radios with the help of the internet. Franchised dealers will also provide you with the code (generally for a fee) on sight of proof of ownership.