Enhance the Pre-Chat Page for Embedded Chat | Embedded Service Chat for Web Developer Guide

Pass Nonstandard Pre-Chat Details
Further control the pre-chat experience using parameters in your Embedded Service code snippet. Two parameters relate to the pre-chat experience: extraPrechatFormDetails and extraPrechatInfo. With these parameters, you can send information to the agent and to your org beyond what’s shown on the pre-chat form.

Follow Pre-Chat Code Examples
During the pre-chat stage: find existing contacts, avoid attaching records to transcripts, attach a record to an existing field, or override a specific field in your org. These examples and more illustrate some common use cases for pre-chat code snippets.

Route Chats Based on Pre-Chat Responses with Direct-to-Button Routing
Set your chat window to route chats to different chat buttons based on the customer’s pre-chat response on any and all of your pre-chat fields. Available when you upgrade your code snippet to version 4.0.

Set Pre-Chat Form Fields to Automatically Populate when Customers Log In
When your customers log in, agents already know basic information like their name and email address. Use this array in your 4.0 code snippet to populate relevant pre-chat fields for them. You can mix and match fields for different record types. This information is for embedded chat windows that are placed outside of Salesforce with Lightning Out (beta). If you use your embedded window in Experience sites, you can enable the contact fields to fill in automatically in the Embedded Chat component settings.