This is a response to the generosity that @Buzinas posted.
This solution will use the NuGet Owin Security Providers package, which is available here .
Go to the package manager console ( Tools -> Library Package Manager -> Package Manager Console) and install the above package:
Install-Package Owin.Security.Providers
In your App_Start\Startup.Auth.csadd the correct instruction using. For steam you want
using Owin.Security.Providers.Steam;
Please note that there are several options, so this package is not only for Steam.

ConfigureAuth app.UseSteamAuthentication("<<API KEY>>");
<<API_KEY>> API, Valve