No Registration without Listserver Opt-In

Here is my challenge/issue:

When new Free Users register for my site I want to require that they opt-in to my listserver (mailchimp), before they can get their username and password for the site.

Right now, when a new user registers for the site, they get a username and password email immediately from wordpress, and they also get an opt-in email from mailchimp.

The new user can choose to opt-in or not to the list-server, but still has access to the site regardless.

Is there a way to do this with S2 Member? Or must I first have them register through mailchimp, and then have them register all over again through S2 Member?

Anybody feel my pain?

I use the S2 Pro

https://www.ads-software.com/plugins/s2member/