Note that the Nook app will not appear in Microsoft Store outside of the US. Here's a method you can use to get it:
https://www.makeuseof.com/download-m...crosoft-store/
It does complain that the application is discontinued but so far I've been able to download all my books with it.
For the key: Get the latest version of DeDRM (the noDRM fork) and check the settings. It can import your key from the app.