There is not a specific API for doing this. This was discussed in the other Connect forum here, Adobe Connect User Community.
For you call I would make the second part differently. Instead of 'field-id=meeting-passcode&value=MY_PASSCODE' I would call 'meeting-passcode=MY_PASSCODE' and see if that worked better.
It's a pity.
Unfortunately, your call didn't work too.
I'm going to find another way. Thank you!