data from form -> modification -> data back to form

Dear Bob,

I would like to get the id of the record on "Site"'s tab "On submit" part by Custom code.
I would like to modify it and save it to an other field of the record what is already in the form but hidden on "Designer" tab.
The field exists in the record but it has no value.

I want to use this code in "Custom code" before database save:

<?php
$orderid = $form->data['id'];
$orderid = "DEMO" . $orderid;
$form->data['orderid'] = $orderid;
?>

How can I get it work?

Thank You,
Andrew