Custom `extract_<slot_name>` method

Rasa Open Source
1

I’m using rasa-x 0.38.1, rasa 2.4.3, and rasa-sdk-2.4.1. I have a form like this:

form_1:
    slot_1:
          ...
    slot_2:
          ...
    slot_3:
          ...
    symbol_name:
    - type: from_entity
      entity: symbol
    - not_intent: intent_1, intent_2, intent_3, intent_4
      type: from_text

I want to define extract_symbol_name method instead of the mapping that we have in the domain file but I don’t know how to do that. How to define these mappings in extract_symbol_name method? How would you define a rejection if these mappings are not met?