How to masquerade while using conversations api

[LBotting]

Hi All!

I'm trying out conversations api & I want to send a message on behalf of another user, like a faculty member, to a student.  I read somewhere that I could use as_user_id = sis_user_id:12345  however my formatting must be incorrect because keeps erroring 

"errors": "Invalid as_user_id"

Is it this format, or is there another id I can try?

Very much appreciate any help you may provide!!