HTTPS / SSL error in subscribe form post

Hi just move my site to use SSL / HTTPS and i cannot validate my site due to:

“Mixed Content: The page at ‘https://www.cooleststufftobuy.com/’ was loaded over a secure connection, but contains a form which targets an insecure endpoint ‘http://cooleststufftobuy.us11.list-manage.com/subscribe/post?u=f70ce4f55d3374e53366475d4&id=a80ba56a9d’. This endpoint should be made available over a secure connection.” => Error.

How i can solve this??
Maybe with “https://www.ostraining.com/blog/coding/mailchimp-forms-ssl/” but how i can do it with the plugin?

Thanks in advance.

https://wordpress.org/plugins/mailchimp-for-wp/