public function sendPreApproval(array $paymentSettings)
    {
        if (empty($this->billingAddress)) {
            $this->setBillingAddress([]);
        }
        $this->validatePaymentSettings($paymentSettings);

        $data = $this->formatPreApprovalData($paymentSettings);

        return (string) $this->sendJsonTransaction($data, $this->url['preApproval'])->code;
    }