Examples of matrix-send-event usages.

Only logged in users can send "files" (images) in a room
When missing a profile-id (of value room_id or room_alias)
With a profile-id, it uses the default form for m.room.message event type (the only event type defined by defaut).
With a custom children HTML form, for custom event type io.gitlab.sctlib.mwc
With a custom form and event type io.gitlab.sctlib.track
with is-widget set to true, does not send to API