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.
Both [Flow (example)](https://www.mediawiki.org/wiki/User_talk:Mattflaschen-WMF?topiclist_preloadtitle=Preloaded%20title&topiclist_preload=User:Mattflaschen-WMF/Preloaded%20body%20example) 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:
1. 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).
2. 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.