To prefill a dropdown question you’ll need to set the query parameter value equal to the answer text. For example, the link below correctly prefills a dropdown custom question:

https://calendly.com/tcampb31-lio/30min/2024-02-28T10:30:00-05:00?a1=answer%201&month=2024-02&date=2024-02-28

Unlike radio button / checkbox questions, dropdown questions cannot be prefilled by passing in a specific option number unless it matches the answer text.

here is no way to find difference between field types in your event types api response.

For example dropdown and  radiobutton both type is single_select 

any update?