This is not for api.php actions. It's for when a script (see e.g. T59989#1499862) wants to prefill a topic title and body without actually saving it.
Both Flow (example) and [old-style talk pages (example)](https://en.wikipedia.org/wiki/User_talk:Mattflaschen-WMF?preloadtitle=Preloaded title&preload=User:Mattflaschen-WMF/Preloaded body example&action=edit§ion=new) support this in basically the same way, but with different URL parameters (topiclist_preloadtitle and topiclist_preload vs. preloadtitle, preload, action=edit, section=new). However, that means they need to detect which it is, which is a pain.
I see two main options:
- Add a new method to mw.messagePoster.MessagePoster. e.g.
function prefill( subject, bodyTemplate)
That would navigate the user to the appropriate URL (or potentially just return that URL).
- Have Flow polyfill (could just be a redirect) index.php?action=edit§ion=new&preloadtitle=&preload.
I think #1 is more consistent with what we did for the post use case, but this is somewhat a different issue.