Hello Pramod, While Constant Contact does not directly provide or support any Drupal modules, search results on drupal.org do appear to show other third-party modules available which support Drupal 8.
If you're able to add Javascript directly to your site, another option would be to use a Constant Contact Inline or Pop-Up Sign-Up form, rather than any Drupal module.
If our own sign-up forms don't quite meet your needs, it may be possible to collect sign-ups using a form created through Drupal, then use a 3rd party such as Zapier to add the contact to your Constant Contact account.
Hopefully one of these options will meet your needs!
Regards,
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