How to implement the OAuth 2 Authorization in C#?

I'm useing the V3 API for a C# ASP.net MVC project. I'm looking to add contact to a list, Is there example code for C# that might be around, how to implement the OAuth 2 Authorization, according to step 1 from https://v3.developer.constantcontact.com/api_guide/server_flow.html,
how to get the redirect url with the access_token, and how to get the authorization code, and refresh token?

and what is "Your app client secret" from Constant Contact .NET SDK API v. 3(https://github.com/mikeks/constantcontact-v3), how to get it. if you send an email, please also cc it to [email protected], thanks.