With double opt-in, after payers check out, they will receive an email with a subscription confirmation link they must click in order to be added to your Mailchimp Audience list. 

1) Navigate to the Extensions tab in the MoonClerk dashboard and click on the MoonClerk Pro for Mailchimp extension.

2) In the Forms section, click on the form whose connection settings you’d like to edit.

3) In the Double Opt-in section, choose Yes.

4) Click Save.