Windows Central posted You do not need to Activate Windows 10 to install it, but this is how you can activate later

A quote from the article:

Microsoft has done an interesting thing with Windows 10. Besides giving it as a free upgrade to anyone with an activated Windows 7 or Windows 8 PC the OS is also one big shareware demo. Specifically, by the demo I mean near fully functional OS that you can install anywhere to try out. This ability means you can download the Windows 10 ISO right from Microsoft and install it on a home-built PC, or any PC for that matter. Even Mac users can do this and install the OS using Bootcamp (more on this later).

 You do not need to Activate Windows 10 to install it, but this is how you can activate later @ Windows Central