Current State:

When using a user-picker field on forms, it is currently not possible to add (create) new customer account via the custom user-picker filed.

However, when the same user-picker is used in a request type that doesn't involve a form, the field allows creating a new customer account.

Proposed Future State:

Add the ability to create a new customer account using custom user-picker fields in forms

Workaround

Remove the user-picker field from the form and add it to the request type