$subscription = new Paymill\Models\Request\Subscription();
$subscription->setClient('client_81c8ab98a8ac5d69f749')
             ->setOffer('offer_40237e20a7d5a231d99b');
             ->setPayment('pay_5e078197cde8a39e4908f8aa');
             ->setPeriodOfValidity('2 YEAR');
             ->setStartAt(1400575533);

$response = $request->create($subscription);