The Hidden Field macro is a mechanism for creating a hidden field whilst making it simpler to use other fields.
Create your form using the Forms for Confluence macro.
Add the Hidden Field macro in the body of the Forms for Confluence 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 box in the order fields will appear in the Response.
When you the page, the field is not visible, but the Response will contain the value given in the macro.