I'm just trying this out for the first time on enwiki. The edit box is preloaded with "Description". That's a little confusing, since "description" makes me think of edit summaries. Maybe change that to "Type you message here"?
Topic on Talk:Talk pages project/New topic/Flow
Type your message here kind of implies the user should type their reply in it though.
The ==Section heading== box says "Title". The main body/text of the message says "Description". I think that "Type your message here" has some potential. @JKlein (WMF), is one of the things you looked at in your user research?