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How can I make an HTTP request and send some data using thePOSTmethod?
In .net core you can make post-call with following code, here I added some extra features to this code so can make your code work behind a proxy and with network credentials if any, also here I mention that you can change the encoding of your message. I hope this explains all and help you in coding.
HttpClient client = GetHttpClient(_config);
        if (headers != null)
        {
            foreach (var header in headers)
            {
                client.DefaultRequestHeaders.TryAddWithoutValidation(header.Key, header.Value);
            }
        }
        client.BaseAddress = new Uri(baseAddress);
        Encoding encoding = Encoding.UTF8;
        var result = await client.PostAsync(url, new StringContent(body, encoding, "application/json")).ConfigureAwait(false);
        if (result.IsSuccessStatusCode)
        {
            return new RequestResponse { severity = "Success", httpResponse = result.Content.ReadAsStringAsync().Result, StatusCode = result.StatusCode };
        }
        else
        {
            return new RequestResponse { severity = "failure", httpResponse = result.Content.ReadAsStringAsync().Result, StatusCode = result.StatusCode };
        }
 public HttpClient GetHttpClient(IConfiguration _config)
        {
            bool ProxyEnable = Convert.ToBoolean(_config["GlobalSettings:ProxyEnable"]);
            HttpClient client = null;
            if (!ProxyEnable)
            {
                client = new HttpClient();
            }
            else
            {
                string ProxyURL = _config["GlobalSettings:ProxyURL"];
                string ProxyUserName = _config["GlobalSettings:ProxyUserName"];
                string ProxyPassword = _config["GlobalSettings:ProxyPassword"];
                string[] ExceptionURL = _config["GlobalSettings:ExceptionURL"].Split(';');
                bool BypassProxyOnLocal = Convert.ToBoolean(_config["GlobalSettings:BypassProxyOnLocal"]);
                bool UseDefaultCredentials = Convert.ToBoolean(_config["GlobalSettings:UseDefaultCredentials"]);
                WebProxy proxy = new WebProxy
                {
                    Address = new Uri(ProxyURL),
                    BypassProxyOnLocal = BypassProxyOnLocal,
                    UseDefaultCredentials = UseDefaultCredentials,
                    BypassList = ExceptionURL,
                    Credentials = new NetworkCredential(ProxyUserName, ProxyPassword)
                };
                HttpClientHandler handler = new HttpClientHandler { Proxy = proxy };
                client = new HttpClient(handler,true);
            }
            return client;
        }