How to generate the control Number for the ISA segment

I'm using the /build/x12 endpoint to generate X12 messages (e.g., 850 transactions), and the response returns a raw EDI string. However, I'm unable to clearly identify or extract the ISA control number (ISA13) from the response.

Could you please confirm:

1. Whether the ISA13 is auto-generated if not provided?

2. How exactly can we extract or validate the generated control number from the response?