The form GUID (Globally Unique Identifier) is a 128-bit integer number used for identification. The GUID is required when using HubSpot's Forms API, and can also help you identify a HubSpot form embedded on an external page.
To locate your form's GUID:
In your HubSpot account, navigate to Marketing > Lead Capture > Forms. If you have a free HubSpot account, click Marketing > Forms.