Vendor/magento/module-customer/Controller/Account/CreatePost.php

if ($this->getRequest()->getParam('is_subscribed', false)) {
            $this->subscriberFactory->create()->subscribeCustomerById($customer->getId());
        }

<?xml version="1.0"?>
  <config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <preference for="MagentoNewsletterModelSubscriber" type="HarrigoSendyConnectModelSubscriberfactory" />
</config>

<?php
namespace HarrigoSendyConnectModel;

use MagentoCustomerApiAccountManagementInterface;
use MagentoCustomerApiCustomerRepositoryInterface;
use MagentoFrameworkExceptionMailException;
use MagentoFrameworkExceptionNoSuchEntityException;

class Subscriberfactory extends MagentoNewsletterModelSubscriber
{
  public function isSubscribed()
  {
    if ($this->sendySubscribed() == "Subscribed") {
        return true;
    } else {
        return false;
    }
  }


protected function _updateCustomerSubscription($customerId, $subscribe)
  {
     ...
  }
}