I understand that by passing the hubspot user token with a form submit that utm_ fields (e.g. utm_source, etc.) can be auto-populated. The fields are on the Contact, on the Hubspot form, and are included in the json body as name/value pairs… so should the value just be set to empty string for those fields?
Hey, @HubspotTEJ 
Thanks for reaching out. Did you make any progress with your project? If not, can you run a quick test, please? Try making a form submission and include the “blank” values as you mentioned. If you get any errors or an unexpected response, please share it back here. This will give the community more to work with when trying to help.
Thanks! — Jaycee