Short answer
HubSpot's KB: add embedded CTAs to drag and drop marketing emails to track the offer across touchpoints. On pages, you can use up to 100 embedded CTAs per page. External pages need the HubSpot tracking code for CTA tracking.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Reusing one CTA across emails and pages keeps its results in one place.