You can try it out now by installing Windows Insider Preview build 17723 or higher, or wait until later this year when it's available to the general public in Windows 10 version 1809. Of course, because this tech is so new (Google's only recently begun doing the same), it'll be a while before many websites get on board, although Microsoft does say in its launch blog post that it's "working with industry partners on lighting up the first passwordless experiences around the web." It's also made it simple for websites to utilize backwards compatibility with external FIDO U2F security devices, so while it'll take a while for mainstream adoption to take hold, this does mark the beginning of a brand new security experience.