Heja @wiki_willy, right, this is currently not possible. See upstream https://secure.phabricator.com/T12552 linking to https://secure.phabricator.com/T7190

I am not sure how to recognize / trigger the subtask creation after creating the parent task...
Was wondering if Herald offers anything, but "create a subtask" is not available as an action.
Maybe webhooks, but I don't know enough of that...

After doing that somehow, a script somewhere could use the Conduit API to create the subtask. Basically:

• Get the internal phid value of the parent task T01234: echo '{"task_id": 01234}' | /path/to/arcanist/bin/arc call-conduit --conduit-uri https://phabricator.wikimedia.org --conduit-token "cli-xxxxx" maniphest.info | jq -r '.response.phid' (to be used in next command, as value for "parents.add")
• Create new subtask: echo '{"transactions":[{"type": "title", "value":"some summary"}, {"type": "description", "value":"some desc"}, {"type": "priority", "value":"medium"}, {"type":"parents.add", "value":["PHID-TASK-yyyyy"]}, {"type":"projects.set", "value":["DC-Ops"]}, {"type":"status", "value":"open"}]}' | /path/to/arcanist/bin/arc call-conduit --conduit-uri https://phabricator.wikimedia.org --conduit-token "cli-xxxxx" maniphest.edit

No better idea currently. :-/