It seems Flow has not defined any HTML accesskeys for previewing/submitting posts and topics.
The default accesskey "p" should be added to the "Preview" button.
The default accesskey "s" should be added to the "Reply"/"Add Topic"/"Submit changes" buttons when replying to a topic/adding a topic or editing a post respectively.
Docs (I'm not sure which of these are most uptodate):
- https://www.mediawiki.org/wiki/Manual:Interface/Access_keys
- https://meta.wikimedia.org/wiki/Help:Keyboard_shortcuts
- https://en.wikipedia.org/wiki/Wikipedia:Keyboard_shortcuts
- https://www.mediawiki.org/wiki/VisualEditor/Portal/Keyboard_shortcuts
See also T158779: {CTRL} + {ENTER} should post a post in flow