The Hidden field of the Mail input macro has been converted into the new Hidden field macro in Forms for Confluence. This is a simpler mechanism for creating a hidden field whilst making it simpler to use other fields.
Create your Form using the Forms for Confluence macros
Add the Hidden Field macro.
In the macro editor, set a Name for the field
Set a Value for the field - you can set a default value for the field
Save the macro to see a placeholder image of how the field will look on the saved page
On saving the page, the field will not be visible and the Response will contain the value given in the macro.