The feature is not currently available on PC and mobile, but the author of this article was able to access some product pages and buy some. As with any product, buying or pre-ordering requires verification by Windows Hello. After that, Microsoft Store will bring up the order page within the application, which looks like the current Microsoft Store page. The writer tested the purchase on Xbox One Media Remote and the order took place normally.
The purchase process is no stranger to Windows Store users
This is a big step for Microsoft / Windows Store when giving Windows 10 users all the latest products from Microsoft and their partners. It is unclear when this feature will be released to the public but will probably come with Windows 10 Fall Creators Update, scheduled for release on October 17.