I finally ended up activating my GitHub account that I signed up for years ago, and publish my first GitHub project. It is a MVC Web API project that uses Google OAuth for authentication. You can see it here.
Even though a lot of the MVC project templates supposedly come with external authentication fully configured, in practice I found that to be quite untrue (other than the single page app template). I could only get this to work after pulling information from several different blogs. I started off with an empty Web API project and only build pieces I needed as I went.
The other thing I was trying to do was send POST messages using Postman to google for authentication, and that did not work at all for me. I did get an authentication token back from Google, but I kept getting 401 when trying to make a POST call to the authorized function. Possible reasons could be, misconfiguration of Postman (even though I did verify everywhere else, and unauthorized GET was working fine), Google blocking it because it wants a consensual mouse click authorizing app access, or something else. I gave up on Postman and made AJAX calls on a simple HTML page.
So, if you are looking to add external authentication into your .NET MVC Web API, maybe this project on GitHub would be of some use to you!