Update field in “Validations on form submission”

Update field in “Validations on form submission”

Hello, I have a hidden field in my form. I created a workflow that is executed when a record is created and triggered an event in the “Validations on form submission”.

At this point I retrieve an external ID and need to add this value to the hidden field in the form. It seems that the simple assignment field = value; does not work in “Validations on form submission”.

Any idea how to force a value into a field before the record is created?

Best regards