Omnipay\Common\Message\AbstractRequest::setTransactionReference PHP Method

    public function setTransactionReference($value)
    {
        return $this->setParameter('transactionReference', $value);
    }

 public function setUp()
 {
     $this->request = new SoapRefundRequest($this->getHttpClient(), $this->getHttpRequest());
     $this->request->setTransactionReference('123456')->setAmount('400.00');
 }