Add a startup.cs class to your application

Startup page is mostly using to configuration for startup of the application when you are configuration OWIN or the OAuth authorization.

There are two what you can directly add it and  as a class file and decorate it as. OWinStartup:

[assembly: OwinStartup(typeof(WebApiOAuth_1.App_Start.Startup))]

Or you can create startup.cs through the Visual Video by using the below process.

a.       Rt click on App_Start à Add à Class

b.      Now search for OWIN at right search box

Click on that to add to the application.

Either you can use the below link or follow the below steps for adding the stratup.cs

https://docs.microsoft.com/en-us/aspnet/aspnet/overview/owin-and-katana/owin-startup-class-detection