The documentation seems to indicate that I can update a harbormaster build with messages from the build server, but I don't know the json needed to do that.

Right now, all I can do is add the word 'work':

echo '{
  "buildTargetPHID": "PHID-HMBT-6ryh6cvsdfpssgy6wrxf",
  "type": "work"
}' | arc call-conduit --conduit-uri https://phabricator.mycompany.com/  harbormaster.sendmessage

Is there another parameter I can send it for a detailed message about the progress (say "10% complete") or something?