On your Freebox management page, normally, if it's the remote control code? If it's the one to register for the Freewifi hotspot, you need to register on the website via a PC on the freewifi site, and the confirmation code will be displayed on the Freebox.
To access online help at 3244!
But the problem is not "for what purpose" but where to find that damn secret code which is not the 8-character password.