For Mailchimp integration you will first need to get your Mailchimp API key and then add that key to your Spotlightr account. Here's how to do that:
Log in to your Mailchimp account, click on your Name/Profile on the top-right corner then click Account. Then go to Extras menu, then click on API Keys.
Under Your API keys and click on Create A Key
Copy the API key that is created
In your Spotlightr account, click on your Name/Profile ont eh bottm-left and then click on Integrations & APIs
From the menu on the right, select Mailchimp. Give it an Integration Name (doesn't matter what it is, up to you) and then paste in the API Key. Then click Create Integration.
You should get a notification at the top saying Integration Created Successfully. You should also now see it under Connected Integrations
You should now see your Mailchimp Integration as an option in your Optin Overlays and Optin Gate settings.