Short answer
HubSpot's KB: paste the global site tag in the site header HTML and the event snippet in thank you pages' head HTML. If you add the tag manually, don't also install the Google Ads pixel through HubSpot, or it fires twice.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Pick one installation method.
3. Related
Tracking URLs and ad conversion events.