Get Refresh Token in Google OAuth2.0 .NET

Question

I have problem using GoogleWebAuthorizationBroker.AuthorizeAsync to authenticate, and looking at some other questions here I went for GoogleAuthorizationCodeFlow. The thing is to use this method I need to get the refresh token.

I have manage to do that following the steps given in this answer, but I haven't find a way of replicating this steps from inside the code in c#.

Is there some way of getting a refresh token and access token using the Google Api?

This is where I need to use it:

ClientSecrets clientSecrets = new ClientSecrets();
clientSecrets.ClientId = id;
clientSecrets.ClientSecret = secret;
String scope = StorageService.Scope.CloudPlatform;

var token = new TokenResponse { RefreshToken = "???" };
var credentials = new UserCredential(new GoogleAuthorizationCodeFlow(
                new GoogleAuthorizationCodeFlow.Initializer
                {
                    ClientSecrets = clientSecrets
                }),
                "user",
                token);
storageService = new StorageService(new BaseClientService.Initializer()
            {
                HttpClientInitializer = credentials,
                ApplicationName = app,
            });