It's not possible to use Windows Authentication for the web screens.

To ensure that you're setup to use both, ensure that users are setup with Authentication Method set to 'Both'.  Specify a password for each user and you should be good to go (having the web screen user enter their Sage 300 User ID and PW)

Hi Halie Howard as Shane - Ascend Business Solutions says there is no Windows Authentication for the Web Screens presently as there is no way to securely detect and enforce the Windows account the user is logged in with via a Web Browser using Sage 300's current login system.  That said, Sage are overhauling authentication for all their products and I would expect to see their Sage ID system with MFA coming to Sage 300 in an upcoming release.  Rather than just adding Windows Authentication, Sage ID is a full OAuth style overhaul along the likes of Microsoft, Google and Facebook's login systems with integration options to boot.  I can't guess an ETA for you but it does seem a priority of Sage's

This seems to be the most likely solution, thanks for the answer!

That would be a great update, especially given Microsoft's push towards multi-factor authentication as well.