Short answer
HubSpot's KB: add a Measurement ID in settings, or paste the code in site header HTML. Single-page tracking needs Professional or Enterprise. With an opt-in consent banner, GA only runs for visitors who opt in.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Expect GA and HubSpot numbers to differ.
3. Related
Why numbers differ and Tag Manager.