Calling a Web Service in Asp.Net Core by Posting JSON and get response in JSON

Question

I am trying to implement a Web Service call in my Asp.Net Core project using HttpClient and HttpResponseMessage. I have to post a json data to an api and in return api will also send the reponse in json. I have tried the below code but its giving me "statuscode: 411, reasonphrase: 'length required' in authorize.net"

I have also tried explicitly adding Content-Length property of header but also i am getting the same response.

Any help on this will be highly appreciated, as i am stuck from past one day.

    public async Task<IActionResult> CallAuthorizeNetPG()
    {
        using (var client = new HttpClient())
        {                
            var postedData = @"{
                ""authenticateTestRequest"": {
                    ""merchantAuthentication"": {
                        ""name"": ""566Mw6rrjNF"",
                        ""transactionKey"": ""65rs7HeJVnu4n744""
                    }
                }  
            }";

            client.BaseAddress = new Uri("https://apitest.authorize.net/");
            client.DefaultRequestHeaders.Accept.Clear();
            client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));            

            StringContent content = new StringContent(JsonConvert.SerializeObject(postedData), Encoding.UTF8, "application/json");
            content.Headers.Remove("Content-Length");
            content.Headers.Add("Content-Length", postedData.Length.ToString());

            try
            {                    

                HttpResponseMessage response = await client.PostAsJsonAsync("xml/v1/request.api", content);
                if (response.IsSuccessStatusCode)
                {
                    RootObject department = await response.Content.ReadAsAsync<RootObject>();                        
                }
            }
            catch (Exception ex)
            {
            }
        }
        return View();
    }