To add the Darwin tracking code to your Wix website you need premium account. This is because Darwin is installed via a Tag Manager or as "custom code" which are enabled once you upgrade. Learn more about this on the Wix help center.
Login to Darwin's Dashboard and visit "Tracking Code" in the navigation.
Copy the tracking code HTML that you'll find there.
Login to Wix and go to settings in your Wix site's dashboard.
Click the Custom Code tab in the Advanced section.
Once there, run through the following:
Once you've saved your changes, you can verify your install on the Darwin dashboard.