using Microsoft.Owin;
using Microsoft.Owin.Security.Cookies;
using Owin;
using Owin.Security.Providers.Steam;

namespace WebApplication
{
    public partial class Startup
    {

        public void ConfigureAuth(IAppBuilder app)
        {
            //other Config

            app.UseSteamAuthentication("your API key");
        }
    }
}