Drupal 8 and MailChimp
I've recently built a subscriber module with a small piece of custom code. I prefer working with webform handlers instead of the Mailchimp module. 
In a custom module add a folder src/Plugin/WebformHandler and create a file newsletterSubscription.php. Add the following code:

&lt;?php
namespace Drupal\&lt;mycustommodule&gt;\Plugin\WebformHandler;

use Drupal\Core\Form\FormStateInterface;
use Drupal\webform\Plugin\WebformHandlerBase;
use Drupal\webform\webformSubmissionInterface;

/**
 * Form submission handler.
 *
 * @WebformHandler(
 *   id = "newslettersubscription",
 *   label = @Translation("Newsletter subscription"),
 *   category = @Translation("Form Handler"),
 *   description = @Translation("Administers subscriptions in Mailchimp"),
 *   cardinality = \Drupal\webform\Plugin\WebformHandlerInterface::CARDINALITY_SINGLE,
 *   results = \Drupal\webform\Plugin\WebformHandlerInterface::RESULTS_PROCESSED,
 * )
 */

class newsletterSubscription extends WebformHandlerBase {

  /**
   * {@inheritdoc}
   */
  public function defaultConfiguration() {
    return [];
  }

  const MAILCHIMP_API_KEY = 'x'; // see https://mailchimp.com/help/about-api-keys
  const LIST_ID = 'x'; // see https://3by400.com/get-support/3by400-knowledgebase?view=kb&amp;kbartid=6


  /**
   * {@inheritdoc}
   */
  public function submitForm(array &amp;$form, FormStateInterface $form_state, WebformSubmissionInterface $webform_submission) {

    $values = $webform_submission-&gt;getData();
    $email = strtolower($values['e_mail']);
    $first_name = $values['first_name'];
    $last_name = $values['last_name'];

    // The data to send to the API
    $postData = array(
      "email_address" =&gt; "$email",
      "status" =&gt; "subscribed",
      "merge_fields" =&gt; array(
        "FNAME" =&gt; "$first_name",
        "LNAME" =&gt; "$last_name"
      )
    );

    // Setup cURL
    // To get the correct dataserver, see the url of your mailchimp back-end, mine is https://us20.admin.mailchimp.com/account/api/
    $ch = curl_init('https://us20.api.mailchimp.com/3.0/lists/'.self::LIST_ID.'/members/');
    curl_setopt_array($ch, array(
      CURLOPT_POST =&gt; TRUE,
      CURLOPT_RETURNTRANSFER =&gt; TRUE,
      CURLOPT_HTTPHEADER =&gt; array(
        'Authorization: apikey '.self::MAILCHIMP_API_KEY,
        'Content-Type: application/json'
      ),
      CURLOPT_POSTFIELDS =&gt; json_encode($postData)
    ));

    // Send the request
    $response = curl_exec($ch);
    $readable_response = json_decode($response);
    if($readable_response-&gt;status == 403) {
      \Drupal::logger('Mailchimp_subscriber')-&gt;error($readable_response-&gt;title.': '.$readable_response-&gt;detail .'. Raw values:'.print_r($values));
      \Drupal::messenger()-&gt;addError('Something went wrong. Please contact your webmaster.');
    }
    if($readable_response-&gt;status == 'subscribed') {
      \Drupal::messenger()-&gt;addStatus('You are now successfully subscribed.');
    }
    if($readable_response-&gt;status == 400) {
      if($readable_response-&gt;title == 'Member Exists') {
        \Drupal::messenger()-&gt;addWarning('You are already subscribed to this mailing list.');
      }
    }

    return true;
  }
}


Read more on how to enable and use the handler here: How to create a mailchimp subscription with drupal 8 webform and custom handlers (Video)