asp.net web api - How to use PostAsXmlAsync with FomUrlEncoding and KeyValuePairs -

i have following webapi, i'm trying find way simplify such that,

i can use postasxmlasync instead of manually serializing object (post has keyvaluepair xmldata="...

also, on handler response.content.readasasync() failing deserialize xml response server.

    [httppost]     [route("validatepayment")]     public async task<paymentresponsedto> validatepayment(paymentrequestdto paymentrequestinfo)     {         var xmlserializer = new xmlserializer(paymentrequestinfo.gettype());         string transactiondata;          using (var stringwriter = new stringwriter())         {             xmlwritersettings xmlwritersettings = new xmlwritersettings();             xmlwritersettings.omitxmldeclaration = true;              using (var xmlwriter = xmlwriter.create(stringwriter, xmlwritersettings))             {                 xmlserializernamespaces ns = new xmlserializernamespaces();                 ns.add("", "");                 xmlserializer.serialize(xmlwriter, paymentrequestinfo, ns);                 transactiondata = stringwriter.tostring();             }         }          var convergeurl = configurationmanager.appsettings["convergeurl"];         var postdata = new list<keyvaluepair<string, string>> {                                     new keyvaluepair<string, string>("xmldata", transactiondata)                                     };          using (var httpclient = new httpclient())         {             using (var content = new formurlencodedcontent(postdata))             {                 content.headers.clear();                 content.headers.add("content-type", "application/x-www-form-urlencoded");                  httpresponsemessage responsemessage = null;                  try                 {                     responsemessage = await httpclient.postasync(convergeurl, content);                 }                 catch (exception)                 {                     throw;                 }                  return handlevalidatepaymentresponse(responsemessage).result;             }         }     }      private async task<paymentresponsedto> handlevalidatepaymentresponse(httpresponsemessage response)     {         if (response.issuccessstatuscode)         {             return await response.content.readasasync<paymentresponsedto>();         }         else if (response.statuscode.tostring().tolower() == "unauthorized")         {             throw new unauthorizedaccessexception("webservice login failure.");         }         else if (response.reasonphrase.tolower() == "not found")         {             return default(paymentresponsedto);         }         else         {             throw new exception(response.reasonphrase);         }     } }