On a clean first installation, booting directly from the installation media, it will ask for a key.
The first install needs to be an upgrade from a 7 or 8.1 with a legitimate key; it will assign a new key for the 10 automatically, and link it with your MS account or something like that, so that you can then do a clean install and it will reapply that same 10 key automatically.
Or so I heard.