Create a link in hook_mail_alter()

Question

I am using hook_mail_alter to modify the email that the user gets when an administrator creates an account. What I am trying to do is create a link in the email body, the link gets created but it adds a [1] to the link text and after the email text is done. There is the [1] again with the actual link. Is there anyway to actually have the link created in the text instead of having it render at the end? Here is my code:

function my_module_mail_alter(&$message) {
if ($message['id'] = 'user_register_admin_created') {

    $url = Url::fromUri('https://www.google.com');
    $external_link = \Drupal::l(t('Google'), $url);
    $message['body'][] = t("Go to: ");
    $message['body'][] = $external_link;
}
}

Answer 1

That happens because the class normally handling the email sending (PhpMail) uses the following code, in PhpMail::format().

  // Convert any HTML to plain-text.
  $message['body'] = MailFormatHelper::htmlToText($message['body']);
  // Wrap the mail body for sending.
  $message['body'] = MailFormatHelper::wrapMail($message['body']);

In particular, this is the code handling the links. (See the comment before the code.)

  // Replace inline <a> tags with the text of link and a footnote.
  // 'See <a href="https://www.drupal.org">the Drupal site</a>' becomes
  // 'See the Drupal site [1]' with the URL included as a footnote.
  static::htmlToMailUrls(NULL, TRUE);
  $pattern = '@(<a[^>]+?href="([^"]*)"[^>]*?>(.+?)</a>)@i';
  $string = preg_replace_callback($pattern, 'static::htmlToMailUrls', $string);
  $urls = static::htmlToMailUrls();

To send HTML emails, you need a different class implementing MailInterface. The Swift Mailer module implements it, for example, and allows to send HTML emails; it also allows to select in which way to send a message (server SMTP, MTA agent, mail() function).
The Mail System module provide a UI that allows to select the mail plugin used to send emails, so you don't need to mess with the configuration to do it.