Setting MailChimp API:

Go to Dashboard >> MailChimp For WP >> MailChimp >> Setting your API Key then clickSave Changes“. To learn how to get API key, you can refer document of MailChimp.

mailchimp

Create MailChimp Form:

Go to Dashboard >> MailChimp For WP >> MailChimp >> Forms:

mailchimp-form

For more detail, you can read document of MailChimp For WP.