Login to Darwin's Dashboard and visit "Tracking Code" in the navigation.
Copy the tracking code HTML that you'll find there.
Login to Hubspot and click on the settings icon in the main navigation bar.
Go to "Website > Pages" on the sidebar menu.
Select the Domain you want to track.
Add the tracking code in the "Site Header HTML" text field and save.
Once you've saved your changes, you can verify your install on the Darwin dashboard.