public function subscribeAddressTransactions($identifier, $address, $confirmations = 6) {
        $postData = [
            'event_type'    => 'address-transactions',
            'address'       => $address,
            'confirmations' => $confirmations,
        ];
        $response = $this->blocktrailClient->post("webhook/{$identifier}/events", null, $postData, RestClient::AUTH_HTTP_SIG);
        return self::jsonDecode($response->body(), true);
    }