Should be something like (I get "Capability createProject is required to access this resource")
gerrit create-project --require-change-id --owner="user/John Erling Blad" --parent=extensions --description='"MediaWiki extension Pickle"' extensions/Pickle
or replace the owner with a more proper group, like a group extension-Pickle.
This repo will be an upstream repo for an existing one at GitHub, https://github.com/jeblad/Pickle
Note that there is a request for rename the tag #Mediawiki-Extensions-Spec to MediaWiki-Extensions-Pickle
This task is also on mw:Git/New_repositories/Requests.