What information is used to populate Marketo's "Original Referrer?" It looks like it is using the Referrer Header from the form fill out event. Any ideas?
That is partially correct. The Original Referrer will be captured from the first web activity associated to a record, such as a Clicks Link, Visits Web Page, or Form Fillout.
Is it possible to populate this field on a list import? I have a lead vendor that can give me the referrer URL, would love to post it to the original referrer field
Does the lead have to be known before the original referrer can be captured or is Marketo able to pull most recent browsing activity based on the first on-site engagement?
The lead doesn't have to be associated before their Original Referrer is recorded, exactly. But you're on the right track. To be precise, the lead's OR is recorded simultaneously with an associating web activity such as a Filled Out Form. So if they visit Page 1 and Page 2, and fill out a form on Page 2, their OR is Page 2. (Page 1 will also be in their Activity Log.)
The problem with the implementation of OR, as you probably see, is that true first touch is not attributed under many circumstances. And the use of the word "referrer" can be confusing as well (it makes you think of the document/page referrer, but it's more like the "association context." But as long as you know what it signifies (and what it doesn't) you can work with it.