[O-Auth2] Automation for sending gmail - where do I find the threadID?

Hi, I’m trying to do the same as the previous poster.

If someone emails me, I’d like an auto-reply to them. If they reply to the same thread, I’d like to auto-reply to them, using knowledge from my OpenAI assistant (with knowledge base).

I see in the previous answer I can use Oauth2 with threadID.

But, where do I find “threadID”?

Thank you in advance!