Calling clientid and clientsecret from external .config file

I've been reading up on best practices for securely storing clientids and clientsecrets: http://www.asp.net/identity/overview/features-api/best-practices-for-deploying-passwords-and-other-sensitive-data-to-aspnet-and-azure

I have created a secret.config file and stored my secrets in there and have put a reference to the file in <appSettings file="....\secrets.config">

I am using

            app.UseGoogleAuthentication(new GoogleOAuth2AuthenticationOptions() {
                ClientId = "",
                ClientSecret = ""
            });

What I need to do is somehow call the clientid and clientsecret but I can't find anywhere how to access it from the external file that I created. Would anybody know how to do this? Or know of a better way of doing this?