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