An sample method for Typetalk api with C# (https://developer.nulab-inc.com/docs/typetalk)

Sample.cs

// Refer follwing modules:
//  using System.Collections.Generic;
//  using System.Net.Http;
//  using System.Web.Script.Serialization;

private async void Application_Startup(object sender, StartupEventArgs e)
{
    var clientId = "xxxxxxxxxxxx";
    var clientSecret = "xxxxxxxxxxxxxxx";
    var topicId = 0; //use your topic id

    var client = new HttpClient();

    var content = new FormUrlEncodedContent(new Dictionary<string,string>() {
        { "client_id", clientId },
        { "client_secret", clientSecret },
        { "grant_type", "client_credentials" },
        { "scope", "topic.post" }
    });

    await client.PostAsync("https://typetalk.in/oauth2/access_token", content).ContinueWith(res =>
    {
        var str = res.Result.Content.ReadAsStringAsync().Result;
        var dic = new JavaScriptSerializer().Deserialize<Dictionary<string,string >> (str);
        var accessToken = dic["access_token"];
        client.DefaultRequestHeaders.Add("Authorization", "Bearer " + accessToken);
        return client.PostAsync("https://typetalk.in/api/v1/topics/" + topicId,
            new FormUrlEncodedContent(new Dictionary<string,string>() { { "message", "Hello, Typetalk!" } })).Result;
    });
}