On chat message, you want to get chat message, session details and visitor details which you can use to keep a repository of chat at your server.
This trigger send JSON data to your web server, as show following.
Example Webhook
{"id":"104111","thread_id":"-1","account_id":"10111","session_id":"61111","room_id":"9395ad5a....","visitor_id":"61111","message":"{"type":"message","msg":"sadsdd","user":{"id":15111,"name":"John","type":"agent","photo":null,"state":"online"}}","recording":"[]","chat_wait_time":"null","chat_duration":"null","translate_to":"en","account_translate_language":"","translate_form_show":"false","date_created":"2018-07-23 05:17:40","date_updated":"2018-07-23 05:17:40","date_picked":"2018-07-23 05:17:42","event":"CHAT_ON_MESSAGE"}