Use the Hidden
field to send ‘hidden’ information along with each form response, useful for marketing purposes, metadata, tracking or anything you need.

You can then click “Edit” on the Hidden
field and configure the Key and Value respectively:

The Key and Value are not visible to the user in your form, and are fully hidden. They are then sent through to your response email when it arrives in your inbox, as well as sent to any plugins.
Common Options
Available on most fields, including this one:
Custom Name | Every field has a unique hidden "name" that can be customized |
Default Value | Set a default value for the field |