I have an ASP.NET Web Api solution which doesn\'t contain a Startup.cs class. I presume this is because the solution wasn\'t created as an MVC solution.
All the code for
Startup.cs is a part of the OWIN authorization package. If the package isn't added through NuGet, I can't guarantee it would work. However, judging by this answer, it might work anyway depending on your environment.
https://stackoverflow.com/a/24678109/6442626
Short answer: If you installed Microsoft.Owin.Security.OAuth from NuGet, that should be good. Otherwise, you need to install it.
Update: In order to get MVC to call the Configuration method in startup, you also need to install the Microsoft.Owin.Host.SystemWeb package from NuGet. There is nothing special you need to change with web.config, IIS will automagically detect the Owin host and load it for you.