You can download
WEBSOCKET API TRY IT NOW! DOCUMENTATION
It contains the web UI tool to test connecting to your ADS and it also provides details about JSON request and response messages you need to use in your app.
We also provide an example on GitHub
To connect to TREP, you need TREP (ADS server) version 3.2.1 and above with the WebSocket enable. I suggest you check with your local Market Data team to verify your TREP version/configuration.
In the mean time, I suggest you check the following Elektron WebSocket API Quick Start and Tutorials: