HttpClient client = new HttpClient();
        string myUser = "my user";
        string myPass = "my pass";
        string clientId = "my client id";
        string redirectUri = "my redirect uri";

        FormUrlEncodedContent form = new FormUrlEncodedContent(new Dictionary<string, string> {
            { "username", myUser }, { "password", myPass }
        });

        Task<HttpResponseMessage> message = client.PostAsync("https://public-api.wordpress.com/oauth2/authorize?client_id=your_client_id" +
            clientId + "&redirect_uri=" + redirectUri + "&response_type=code", form);
        String result = message.Result.Content.ReadAsStringAsync().Result;

        JObject obj = JObject.Parse(result);
        string token = (string)obj["access_token"];
        string refreshToken = (string)obj["refresh_token"];

Invalid request, please go back and try again.
Error Code: invalid_client
Error Message: Unknown client_id