When a Squarespace 6 form is connected to a Mailchimp list, the structure of the form is mirrored to the list's data configuration. Each Squarespace form field is mapped to an equivalent Mailchimp list field automatically as the form is built. Form submissions are then captured in Mailchimp as list subscribers. This integration makes it easy for Squarespace users to add Mailchimp newsletter signup forms to their sites.
Additional Information & Help
Using Mailchimp with Squarespace
Troubleshooting Mailchimp issues
Squarespace + Mailchimp Integrations