As a HubSpot Marketing Free or Starter customer, it is not currently possible to create a follow up email for Lead Flow or Collected Form submissions in HubSpot.
Lead Flows and Collected Forms currently integrate with MailChimp. In MailChimp, you can create an autoresponder to send an email to any new contact added to a MailChimp list through a Lead Flow or Collected Form. Read more about configuring HubSpot and MailChimp here.
Please Note: If you are a HubSpot Marketing Professional or Enterprise customer, you can create a Workflow to enroll contacts based on 'Form Submission' to send your contacts an automated email from HubSpot after a submission on your Lead Flow or Collected Form.