Authorization

Click Configure, then Show Panel on the right bar, and finally Sign in with Mailchimp.

Mailchimp Block- Configure

Click the Authorize button.

Mailchimp Block- Authorize Button

Show the login page, then click the login button.

Mailchimp Block- Login

All okay, then click the Allow button.

Mailchimp Block- Allow Your Mailchimp Account

All right, then Authentication success message appears.

Mailchimp Block Success