>> EDIT: it might even be as simple as the fields on both sites are called
>> 'subject' but I doubt it.
Actually I think it might be as simple as this, although note that the field name doesn't come from what is displayed on the page.
If you view the page source (in my Firefox that's View/Page Source) and search for 'subject', you can see the source for the displayed text 'Subject:'. Shortly after that is 'name="subject"', and it's the 'name="X"' which (I think) identifies the field as X for auto-complete.
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