Short answer
HubSpot's KB: the GUID is required for the Forms API. In the updated editor it's the data-form-id value in the embed code; in legacy forms it's formId. It also appears in the editor's page URL.
1. In HubSpot
The relevant screen in my demo portal.
2. Worth knowing
Store GUIDs with your integration config so forms aren't mixed up.
3. Related
Forms submission API and reading submissions.