Step 1:

Go to the ‘Settings’ tab and select ‘Integrations’.

Step 2:

Tick the ‘Checkbox’ to enable ‘Opt-In’. Copy the ‘API key’ from your MailChimp account and paste it into the ‘API Key’ field. Your ‘API key’ can be found by going to https://us1.admin.mailchimp.com/account/api/.

Step 3:

Copy the ‘List ID’ from your MailChimp account and paste it into the ‘Unique List ID’ field. The ‘List ID’ can be found by clicking on your list in MailChimp and then the settings page inside the chosen list.

Step 4:

Select ‘Save’. The sign-in form will now have an additional checkbox (highlighted below) to ‘Opt-In’.