Hello there

Our project aims to create a marketplace app for Zendesk chat.

Our objective is to automatically give a response to users based on their inquiries whenever they post messages using the web chat widget.

First, we attempted to accomplish this by manually setting up a webhook. 
Conversations Integrations (Webhook subscriptions - Conversation Message) can be found in Zendesk Admin > Apps & Integrations> Integrations > Conversations. 
With this approach, we have accomplished our objective.

Then, we began to develop an app (which runs continuously in the background and has NO UI). But we were unable to locate a suitable event.