First, add your API key under Holler Box => Settings.
You can find your API key in your MailChimp account under Account => Extras => API Keys. Generate a new one if it doesn’t exist.
Save your API key.
Next, in your Holler Box, choose Mailchimp as the opt-in provider. Add your list ID. You can find this under Lists => Your List => Settings => List name and defaults. Look on the right side of the screen for List ID.
Copy/paste that into the MailChimp list ID field and save.
When you create a new Holler Box and choose MailChimp for your provider, you need to add your list ID. This is the list new subscribers will be added to.
Get your list ID in your MailChimp account under Lists => Settings => List name and defaults => List ID (on right side of screen)