embeddedservice_bootstrap.prechatAPI.setVisiblePrechatFields() can be used to auto populate Pre-Chat Form Fields in Salesforce Messaging for In-App and Web.

Sample Code:

window.addEventListener("onEmbeddedMessagingReady", e => {
    embeddedservice_bootstrap.prechatAPI.setVisiblePrechatFields({
        "_firstName": {
            "value": "Test",
            "isEditableByEndUser": false
        },
        "_lastName": {
            "value": "User",
            "isEditableByEndUser": false
        },
        "_email": {
            "value": "[email protected]",
            "isEditableByEndUser": false
        },
        "_subject": {
            "value": "Testing",
            "isEditableByEndUser": true
        }
    });
});

Use the above code before the embeddedservice_bootstrap.init() method.

isEditableByEndUser indicate wheter the Pre-Chat field can be updated by the Visitor or not.

Output: