code: protected function loadCustomerRefs($dkNumber)
      {
          if (!is_null($dkNumber)) {
              $this->customerRef = new MasterPricer\CustomerRef();
              $this->customerRef->customerReferences[] = new MasterPricer\CustomerReferences(
                  $dkNumber,
                  MasterPricer\CustomerReferences::QUAL_AGENCY_GROUPING_ID
              );
          }
      }
docs: Load Customer references
  
  @param string $dkNumber