code: private function createRequest()
      {
          $xml = new DOMDocument();
          $xml->formatOutput = true;
  
          $trackRequest = $xml->appendChild($xml->createElement('TrackRequest'));
          $trackRequest->setAttribute('xml:lang', 'en-US');
  
          $request = $trackRequest->appendChild($xml->createElement('Request'));
  
          $node = $xml->importNode($this->createTransactionNode(), true);
          $request->appendChild($node);
  
          $request->appendChild($xml->createElement('RequestAction', 'Track'));
  
          if (null !== $this->requestOption) {
              $request->appendChild($xml->createElement('RequestOption', $this->requestOption));
          }
  
          if (null !== $this->trackingNumber) {
              $trackRequest->appendChild($xml->createElement('TrackingNumber', $this->trackingNumber));
          }
  
          if ($this->isMailInnovations()) {
              $trackRequest->appendChild($xml->createElement('IncludeMailInnovationIndicator'));
          }
  
          if (null !== $this->referenceNumber) {
              $trackRequest->appendChild($xml->createElement('ReferenceNumber'))->appendChild($xml->createElement('Value', $this->referenceNumber));
          }
  
          if (null !== $this->shipperNumber) {
              $trackRequest->appendChild($xml->createElement('ShipperNumber', $this->shipperNumber));
          }
  
          if (null !== $this->beginDate || null !== $this->endDate) {
              $DateRange = $xml->createElement('PickupDateRange');
  
              if (null !== $this->beginDate) {
                  $beginDate = $this->beginDate->format('Ymd');
                  $DateRange->appendChild($xml->createElement('BeginDate', $beginDate));
              }
  
              if (null !== $this->endDate) {
                  $endDate = $this->endDate->format('Ymd');
                  $DateRange->appendChild($xml->createElement('EndDate', $endDate));
              }
  
              $trackRequest->appendChild($DateRange);
          }
  
          return $xml->saveXML();
      }
docs: Create the Tracking request.
  
  @return string