Why Call to WebAPI using C# HttpClient not working even if it is working in postman. what am i doing wrong

Here is WebAPI EndPoint: https://mysoylplus-apptest.frontierag.co.uk/api/someendpoint

i am sending formData in postman in key value pair like below: authenticationData[b64EncryptedSoylSenseInfo] : 'somekey', authenticationData[b64iv] : 'some key' the above data is in key value pair.

the above thing is working in postman but when i use HTTPclient in c# code. this is giving me 401 exception.

What am i doing wrong , using postman i am getting 200 ok, but using below i am geting 401. Please help

string url = "https://mysoylplus-apptest.frontierag.co.uk/api/someendpoint";

        using (var client = new HttpClient())
        {
            client.DefaultRequestHeaders.Add("Accept", "application/x-www-form-urlencoded");                



            using (var httpContent = new HttpRequestMessage(HttpMethod.Post, url))
            {
                client.Timeout = TimeSpan.FromMinutes(20);

                var values = new Dictionary<string, string>();   

                values.Add("authenticationData[b64EncryptedSoylSenseInfo]", "somekey");
                values.Add("authenticationData[b64iv]", "somekey");


                httpContent.Content = new FormUrlEncodedContent(values);

                var response = client.PostAsync(url, httpContent.Content);

                // here is the hash as string
                var result = response.Result.ToString();
            }
        }