Consume WEB API in CRM plugin returning "Server Unavailable"

Hi guys, i want to consume WEB API from Dynamics CRM plugin, but returning "503: Server Unavailable"

using (MemoryStream serializeAssignmentWO = new MemoryStream())
    {
        AssignmentWO std = new AssignmentWO();
        std.AssignmentWONo = "123457";
        std.AssignmentDate = "2021-06-08";
        std.CustAcct = "10000514";
        std.Address = "Karawaci";
        std.TapValue = "Test";
        std.Longitude = 0;
        std.Latitude = 0;
        std.CableDropPortNo = 0;
        std.FixTemp = "Test";

        DataContractJsonSerializer serializer = new DataContractJsonSerializer(typeof(AssignmentWO));

        WebRequest request = WebRequest.Create("http://bmjkt-000037:9000/crmapi/api/RequestWONoise");
        request.Method = "POST";
        request.ContentType = "application/json";
        request.ContentLength = 0;

        serializer.WriteObject(serializeAssignmentWO, std);


        StreamReader sr = new StreamReader(serializeAssignmentWO);

        string ser_result = sr.ReadToEnd();

        throw new InvalidPluginExecutionException("Result: " + request.GetResponse());
    }
public class AssignmentWO
    {
        [DataMember]
        public string AssignmentWONo { get; set; }
        [DataMember]
        public string AssignmentDate { get; set; }
        [DataMember]
        public string CustAcct { get; set; }
        [DataMember]
        public string Address { get; set; }
        [DataMember]
        public string TapValue { get; set; }
        [DataMember]
        public int Longitude { get; set; }
        [DataMember]
        public int Latitude { get; set; }
        [DataMember]
        public int CableDropPortNo { get; set; }
        [DataMember]
        public string FixTemp { get; set; }
        }

If i test using Fiddler, its returning: success