[RoutePrefix("client")]
controller
        [HttpPost]
        [Route("find-client-by-xml")]
        public async Task<IHttpActionResult> FindClientByReqs(FindClient request)
        {
            ...

            return Ok();
        }

    [XmlRoot("find")]
    public class FindClient
    {
        [JsonProperty("tax_number")]
        [XmlElement("TaxNumber")]
        public string TaxId { get; set; }

            HttpWebRequest request = (HttpWebRequest)WebRequest.Create("http://localhost:60308/client/find-client-by-xml");
            byte[] bytes;
            bytes = System.Text.Encoding.ASCII.GetBytes("<TaxNumber>123</TaxNumber>");
            request.ContentType = "text/xml; encoding='utf-8'";
            request.ContentLength = bytes.Length;
            request.Method = "POST";
            Stream requestStream = request.GetRequestStream();
            requestStream.Write(bytes, 0, bytes.Length);
            requestStream.Close();

            HttpWebResponse response;
            response = (HttpWebResponse)request.GetResponse();

  
<Error>
   <Message>Запрос недопустим.</Message> 
 <ModelState>
   <request>В документе XML (1, 12) присутствует ошибка.</request> 
 </ModelState>
</Error>

  <?xml version="1.0" ?> 
 <find xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <TaxNumber>123</TaxNumber> 
  <RegistrationCountry>Count</RegistrationCountry> 
 </find>

- <Error>
  <Message>An error has occurred.</Message> 
     <ExceptionMessage>Ссылка на объект не указывает на экземпляр объекта.</ExceptionMessage> 
     <ExceptionType>System.NullReferenceException</ExceptionType> 
  </Error>

       [HttpPost]
        [Route("find-client-by-xml")]
        public IHttpActionResult GetClient(FindClientByReqsRequest request)
        {
            return Ok(request.TaxId);
        }

            XmlMediaTypeFormatter xml = GlobalConfiguration.Configuration.Formatters.XmlFormatter;
            xml.UseXmlSerializer = true;