Because Microsoft offers OneNote as a part of its Office suite, it is not possible to find a product key for OneNote only. Likewise, Office is a licensed product and requires subscription. However, a free version of OneNote that does not ask for a product key is also available.
The free version of OneNote is available for Windows, Mac OS, Chromebook, iOS and Android devices. It comes preinstalled with Windows 10. Type OneNote in the search bar on the bottom left, and click OneNote, Trusted Windows Store App to launch the free version.
OneNote is an advanced note-taking application that allows the user to clip content from the Web, snap photos, take notes with a stylus or touch input, and work together on a file with other people. Notes taken on one platform also show up on all others.