Bullhorn requests you contact them via support to set up a REST OAuth App.
Contact Bullhorn Support.
Bullhorn will send you credentials, as well as, an API key and secret (client_idand client_secret). They also require some additional information regarding your app.
Bullhorn follows standard OAuth2 from there.
Find out more information about the Bullhorn API and Support Forum.
Once you have your apiKey, apiSecret, and callbackUrl please create an instance.