USPS Track Api XML不工作ios

Question

跟踪通过Priority Mail Express™，GlobalExpressGuaranteed®或Priority Mail Express International™服务发运的任何包裹。查看USPS Tracking™，Signature Confirmation™，CertifiedMail®或Registered Mail™服务附带的软件包的跟踪信息。

我是FedEx，UPS和USPS提供的装运API的新程序员。我已成功使用fed ex和UPS XML API来获取Package的TRACK细节。但是，当我尝试使用此https://www.usps.com/business/web-tools-apis/delivery-information.htm上给出的USPS跟踪API XML IE时。 我遇到了一些问题......这是代码的一部分..

URL

 NSURL *url = [NSURL URLWithString:@"http://production.shippingapis.com/ShippingAPITest.dll?API=TrackV2&"];
    request = [NSMutableURLRequest requestWithURL:[url standardizedURL]];
    tracknum=@"EJ958083578US";
    NSString *userid=@"XXXXXXXXXX";
    xml_req =[NSString stringWithFormat:@"XML=<TrackRequest USERID='%@'><TrackID ID=\"%@\"></TrackID></TrackRequest>",userid,tracknum];



[request setValue:@"application/xml; charset=utf-8" forHTTPHeaderField:@"Content-Type"];
    [request setHTTPBody:[xml_req dataUsingEncoding:NSUTF8StringEncoding]];    

然后我得到了这个问题..

     Error =     {
                Description ={
                    text = "XML Syntax Error: Please check the XML request to see if it can be parsed.";
                };
                Number ={
                    text = 80040B19;
                };
                Source ={
                    text = "USPSCOM::DoAuth";
                };
            };
        }

谁能告诉我为什么会这样？我应该怎么做才能得到适当的回应。？