That bried tour of the api is an old article regarding 1.7 API.
For 1.8 please see http://xibo.org.uk/manual-tempel/en/api.html
and my API Introduction with Postman
Yes, client_secret is rather long 
You should find asnwers to your question in manual pages and/or my API introduction topic.
If you’d have more questions, please feel free to ask.