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.
In your HubSpot account, navigate to Marketing > Lead Capture > Forms. If you have a free HubSpot account, click Marketing > Forms.
Click the name of the form.
In the upper right, click Share.
In the dialog box, look for the formID attribute value in the embed code, which is the form GUID. The form GUID also appears in the page URL when you're in the form editor.