What's the correct embedded_svc.settings.extraPrechatInfo for this simple use case ?

We would like to have a simple embedded prechat form with firstname, lastname, email address and subject, mapped to the usual Contact and Case objects.

However, we would like to use only the email address as a key to search for an existing similar contact; if it does not exist, we would like to create with all 3 fields first name, last name, email address.

Lastly, an option would be to update the first name and lastname fields of an existing contact found with the same email address.

The documentation is not clear enough for me here :

https://developer.salesforce.com/docs/atlas.en-us.238.0.snapins_web_dev.meta/snapins_web_dev/snapins_web_prechat_details.htm

and here :

https://developer.salesforce.com/docs/atlas.en-us.238.0.snapins_web_dev.meta/snapins_web_dev/snapins_web_prechat_code_examples.htm

Thanks.