To add the Darwin tracking code to your Squarespace website you need premium plan.
Login to Darwin's Dashboard and visit "Tracking Code" in the navigation.
Copy the tracking code HTML that you'll find there.
Login to Squarespace and go to settings in your Squarespace site's dashboard.
Navigate to "Advanced > Code Injection".
Add the tracking code into the "Header" text field and save changes.
Once you've saved your changes, you can verify your install on the Darwin dashboard.