Create hidden custom form fields on all funnel forms

Hello! I'm trying to use the API to automatically create hidden inputs in all forms. I have used the calls from /api/v2/workspaces/<workspace_id>/forms and then the one from /field_sets with the form_id but I don't quite understand this field_sets that appears to me which form of the funnels I have it refers to.

The forms response returns me "Custom Contact Attributes" and the field_sets returns "Default Field Set"

When I try to get the fields inside the field set I get a 404 error.

Thank you!