The new paramter initial_decentro_transaction_id will be live after 20th December 2022. Please try after 20th December 2022, not before that with the above parameter.
This API is used to get Access Token from Digilocker. Refer here for Access Token Documentation
DigiLocker Code received on the UI will have to be passed in the request body parameter digilocker_code
DigiLocker Response Keys
Here are the Response Keys and the scenarios under which Decentro will propagate the same to the platform - Response Keys