Startup.cs

using Microsoft.Owin;
using Microsoft.Owin.Security.OAuth;
using Owin;
using System;
using System.Threading.Tasks;
using System.Web.Http;

[assembly: OwinStartup(typeof(WebApplication4.Startup))]

namespace WebApplication4
{
    public class Startup
    {
        public void Configuration(IAppBuilder app)
        {
            app.UseCors(Microsoft.Owin.Cors.CorsOptions.AllowAll);

            OAuthAuthorizationServerOptions options = new OAuthAuthorizationServerOptions
            {
                AllowInsecureHttp = true,

                //The Path For generating the Toekn  
                TokenEndpointPath = new PathString("/token"),

                //AuthorizationServerProvider class will validate the user credentials  
                Provider = new AuthorizationServerProvider()
            };

            //Token Generations  
            app.UseOAuthAuthorizationServer(options);
            app.UseOAuthBearerAuthentication(new OAuthBearerAuthenticationOptions());

            HttpConfiguration config = new HttpConfiguration();
            WebApiConfig.Register(config);
        }
    }
}