client-side scrip to intercept form values

I need a client-side scrip that can be called from a header file which will capture values of specified fields in a form and will send/POST them to another page (either in a hidden IFRAME or in the background) BEFORE the page/form submits and goes to the TARGET of the FORM. ## Deliverables this script will be called independently from the page (probably loaded in the header) and the event trigger must be something similar to a PAGE ONUNLOAD since we can not modify the actual form from which we must extract the field values from.