So... look at what's in the normal post sending to reply to a topic.
When replying to this topic, I see the submit URL is
https://www.simplemachines.org/community/index.php?board=8;action=post2 - which *on its own* would be the post to a new topic submit button (note the action=post2 part)
But when I look at all the other things submitted normally from QR:
topic - the topic ID
subject - the subject for the message
icon - an icon, defaults to 'xx'
from_qr - indicates if from quick reply or not, I forget exactly what that does
notify - whether to turn notifications on for the topic
not_approved - relates to the post approval workflow, leave blank if post moderation is not applicable
goback - 0 to redirect to the list of topics, 1 to redirect back to the topic itself after posting
last_msg - the last msg in the topic that you saw (so it knows whether to check the 'something has been posted while you were typing' warning)
seqnum - a value to check if you already submitted or not
plus the session identifier.
PLUS, and this is the thing I think you're missing:
post - with a value, to indicate you pressed the Post button and not preview.