ASIHTTPRequest post服务器数据求助

我现在想向WCF端的服务器post请求，post请求带参数
参数是：{“UserName”:”zack3″,”Password”:”1111111111″}的json字符串,服务器端参数是解析json

NSURL *url = [NSURL URLWithString:@”http://192.168.3.24/WCFServices/Login
  ASIHTTPRequest *request = [ASIHTTPRequest requestWithURL:url];
  [request startSynchronous];

  [request setPostValue:@”Ben” forKey:@”first_name”];
  [request setPostValue:@”Copsey” forKey:@”last_name”];
  [request setTimeOutSeconds:10];
  [request addRequestHeader:@”Content-Type” value:@”text/xml; charset=utf-8″];

  NSError *error = [request error];
  if (!error) {
    NSString *response = [request responseString];
  }

请问我怎么把这段参数post过去，并且返回的数据不是乱码，急急急