Edit web form fields by participant without own Adobe Sign account?

The scenario is as follows:

• We host a platform with multiple users
• We would like our users to sign agreements with each other via our Adobe Sign account
• We are not part of the signed agreements
• Our users do not have an Adobe Sign account themselves
• Our ideas is:
  • User A uploads a PDF file (i.e. a contract that has to be signed by User A himself and User B)
  • We create an Adobe Sign Web Form / Widget with this PDF file

Important: User A should now be able to add any signature fields they wish to the PDF via the Adobe Sign Web Form without requiring an Adobe Sign account on his own.

For testing, we created the Web Form via REST API and set User A as the initial participant. The Web Form itself is set to the AUTHORING state.

But, upon embedding the widget, in order to edit/add form fields, User A is prompted to sign in with his Adobe Sign account.

How can we let User A add the required signature field(s), finish the web form (so it becomes ACTIVE) and subsequently send it out for signing?