Looking at urlPattern.properties, the only calls seem to be 1) creating a user, 2) listing all users, 3) creating a post. That file also tells you which parameters are required.
"api_key" is apparently something you need to put into the "jforum_api" table. In my local installation I just performed
insert into jforum_api values (nextval('jforum_api_seq'::regclass), 'foobar', '2015-01-01')
and after that the URL
http://localhost:8080/jforum2/userApi/list/foobar.page returned the list of all users.