Hi Pavol,

Correct me if I'm wrong, do you mean you want to send welcome email for newly registered user (eg: http://take.ms/9xDlO ) through mailchimp instead of normal email send out from ES? In mailchimp actually there is welcome email http://take.ms/EMOeN when you enable the opt-in option http://take.ms/xFRpR https://www.screencast.com/t/D618me9eX2m5 where you can edit the welcome email here: http://take.ms/bY4Vi
As the new mailchimp api v3.0 seems there is no way to send welcome email without disabling the opt-in https://stackoverflow.com/questions/35985147/sending-final-welcome-mail-with-mailchimp-api-v3-0-without-double-opt-in

And to be honest I'm not really sure/familiar the automation app like Zapier
After a few research, seems like there is automation in mailchimp itself: https://help.mailmunch.com/hc/en-us/articles/206143089-How-to-Enable-MailChimp-s-Welcome-Email perhaps, you can try it out.