To add the Darwin tracking code to your Webflow website you need a paid site plan. This is because Darwin is installed as "custom code" which is enabled once you upgrade.
Login to Darwin's Dashboard and visit "Tracking Code" in the navigation.
Copy the tracking code HTML that you'll find there.
Login to Webflow, click on the Webflow icon in the top left and go to "Project Settings".
Click on "Custom Code"
Add the Darwin tracking code to the "Head Code" textbox and save changes.
Click "Publish" on your project settings, select the domain for the site listed in Darwin and click "Publish to Selected Domains".
Once you've saved your changes, you can verify your install on the Darwin dashboard.