Reproduce form behavior of 2.0 to 3.0

I have a form that runs this extract_slotform function at the beginning.

That function and a validation function worked well with Rasa 2.0, but when I try to migrate it to Rasa 3.0, it doesn’t get executed anymore.

I’ve already used the migration guide and added the custom mapping as described in the documentation.

Is there a way to force the function call?