embeddedservice_bootstrap.prechatAPI.setHiddenPrechatFields() can be used to pass Hidden Pre-Chat data in Salesforce Messaging for In-App and Web.

If you are new to Messaging for In-App and Web, then use the following link for Pre-Chat Setup.

Pre-Chat in Salesforce Messaging for In-App and Web

1. Create a Custom Field on the Messaging Session Object to store the Hidden Pre-Chat information.

2. Find the Omni-Channel Flow used on the Messaging Channel.

3. In the Omni-Channel flow, create an input variable to get the values from the Pre-Chat form. In my example, I have used Update Records Flow Element to update the Messaging Session record with the Hidden Pre-Chat information.

4. Create a Custom Parameter. Map the Custom Parameter with the Flow Variable Name.

5. Go to the Embedded Service Deployment. Edit the Pre-Chat form. Add the Custom Parameter to the Hidden Pre-Chat Fields.

6. Publish the Embedded Service Deployment.

7. Go to the Experience Cloud Site Builder. Add the following code in the Head Markup.

<script>
	window.addEventListener(
		"onEmbeddedMessagingReady", () => {
			console.log(
				'Inside Messaging Ready Block'
			);
			let userId = $A.get(
				'$SObjectType.CurrentUser.Id'
			);
			console.log( 
				'User Id', 
				userId
			);
			embeddedservice_bootstrap.prechatAPI.setHiddenPrechatFields(
				{
					'User_Id' : userId
				}
			);
		}
	);
</script>

8. Publish the Experience Cloud Site.

9. Test it from the Experience Cloud Site once it is published.

Output: