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 username at the bottom-left corner then go to Profile >> Account & Billing >> Extras >> API Keys.
Click on Create A Key under Your API keys.
Copy the API key that is created.
Log in to Spotlightr, go to App Settings >> Integrations >> Add Integration:
Click on Mailchimp, this will open up a pop-up (pop-ups should be allowed in your browser).
Add any title, paste the API Key into the corresponding field and click on the Create button.
You should get a notification at the top saying Integration Created Successfully and you should also now see it under connected Integrations.
Your Mailchimp Integration will now be available as an option in your Opt-in Overlays and Opt-in Gate settings.