Hidden Form Field dialog box
Develop > Form > Form field > Hidden Form Field
Although not visible to the site visitor, the name and value of the field is sent to the Web server when the site visitor submits the form.
Name
Enter a name for the hidden form field. The name is used to distinguish this field from other fields in the same form.
Value
Enter a value to be sent to the Web server when the form is submitted.
In the document window, hidden form fields are marked with a small yellow icon that looks like a partially-obscured check box.