I'm building the "respondent" part of a survey application and for that I need some sort of authentication framework. It have some special need though:

• The only login option should be a unique code. This code is usually appended to a link in the survey invitation email, but some email setups does not allow direct linking so I need to be able to type/paste the code to login
• There is only one role, the respondent. All management and setup is done in a separate application.

How would you do this? This is a rewrite of an existing application and the old application used "normal" .NET Forms/Cookie auth with a self-rolled UI.

Is this still the best solution? The platform is ASP.NET Core on full .NET 4.6.1 framework. Open to third party libraries as long as they are MIT or similar (no GPL)