Why always get Bad Request in my c# console application but postman is working fine with NTLM authorization

Question

private static string APIUrl = "http://201.881.xxx.xx:1047/UATDB/WS/Leo/Codeunit/WebApi";

public static async Task GetDataWithAuthentication()
{
    var authCredential = Encoding.UTF8.GetBytes("{apitest}:{apitest}");
    CredentialCache credentials = new CredentialCache();
    credentials.Add(new Uri(APIUrl), "NTLM", new NetworkCredential("apitest", "apitest", ""));

    using (var client = new HttpClient())
    {
        client.DefaultRequestHeaders.Accept.Clear();
        client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/xml"));
        client.DefaultRequestHeaders.Authorization =
            new AuthenticationHeaderValue("NTLM", Convert.ToBase64String(Encoding.UTF8.GetBytes(credentials.ToString())));
        client.DefaultRequestHeaders.Add("Soapaction", "urn:microsoft-dynamics-schemas/codeunit/WebApi");

        client.BaseAddress = new Uri(APIUrl);
        string soapString = @"<?xml version=""1.0"" encoding=""utf-8""?>
  <soap:Envelope xmlns=""http://schemas.xmlsoap.org/soap/envelope/"">
      <soap:Body>
          <GetCustomer xmlns=""urn:microsoft-dynamics-schemas/codeunit/WebApi"">
            <customerNo></customerNo>
            <xCustomer>
                <Customer xmlns=""urn:microsoft-dynamics-nav/xmlports/x50007"">
                </Customer>
            </xCustomer>
          </GetCustomer>
      </soap:Body>
  </soap:Envelope>";
        StringContent content = new StringContent(HttpUtility.UrlEncode(soapString), Encoding.UTF8, "application/x-www-form-urlencoded");

        List<KeyValuePair<string, string>> requestData = new List<KeyValuePair<string, string>>();
        requestData.Add(new KeyValuePair<string, string>("grant_type", content.ToString()));
        FormUrlEncodedContent requestBody = new FormUrlEncodedContent(requestData);

        var requestUrl = client.PostAsync(client.BaseAddress, requestBody).Result;
        requestUrl.Content = content;
        if (requestUrl.IsSuccessStatusCode)
        {
            var result = requestUrl.Content.ReadAsStringAsync().Result;
        }
        HttpResponseMessage response = await client.GetAsync(client.BaseAddress);
        response.Content = content;
        if (response.IsSuccessStatusCode)
        {
            var readTask = response.Content.ReadAsStringAsync().ConfigureAwait(false);
            var rawResponse = readTask.GetAwaiter().GetResult();
            Console.WriteLine(rawResponse);
        }
        else
        {
            Console.WriteLine("{0}\n{1}", response.StatusCode, response.RequestMessage);
        }
        Console.WriteLine("Complete");
    }
}