asp.net mvc - enter data in another website and get response back with c# -

i want post data http://irsherkat.ssaa.ir/design/searchcompanypublicinfo.aspx value of example 10861677542 #txtnationalcode input. when use fiddler post request http://irsherkat.ssaa.ir/hndlr/generalhandler.ashx?id=33 (which posted data sent to), request header:

content-type: application/x-www-form-urlencoded 

and request body:

request={'classtypename':'trm.hndlr.classes.companypublicinfocontroller','methodname':'searchdata','dataparamtype':'trm.hndlr.trmtypedefinition.companypublicinfo.companypublicinfoserchparam','mainentitytype':'','dataparam':'{\'nationalcode\':\'10861677542\',\'registernumber\':\'\',\'unitid\':null,\'companytypeid\':\'0\'}'} 

i 200 success result when want c# code:

public async task<string> get()     {         var request = (httpwebrequest)webrequest.create("http://irsherkat.ssaa.ir/hndlr/generalhandler.ashx?id=33");          var postdata = "request={'classtypename':'trm.hndlr.classes.companypublicinfocontroller','methodname':'searchdata','dataparamtype':'trm.hndlr.trmtypedefinition.companypublicinfo.companypublicinfoserchparam','mainentitytype':'','dataparam':'{\'nationalcode\':\'10861677542\',\'registernumber\':\'\',\'unitid\':null,\'companytypeid\':\'0\'}'}";         var data = encoding.ascii.getbytes(postdata);          request.method = "post";         request.contenttype = "application/x-www-form-urlencoded";         request.contentlength = data.length;          using (var stream = request.getrequeststream())         {             stream.write(data, 0, data.length);         }          var response = (httpwebresponse)request.getresponse();          return new streamreader(response.getresponsestream()).readtoend();     } 

it gives me error:

the remote server returned error: (500) internal server error. 

try remove "request=" postdata value.