Tracking, cookies and web to lead forms

Example site:
http://www.myclientsite.com.au/?utm_source=tbf&utm_medium=banner&utm_campaign=brand

Someone comes from that link (as you can see it’s tagged with the source)

Let’s say someone fills out a form AFTER they’ve been on the site for a while (clicked around etc.), how can I still pass the Lead Source as ‘tbf’ (trailerboat fisherman) to the form, do I create a cookie of some sort?

ALSO

If you have a Mac running safari can you tell me, do the drop downs work for this:
DropDowns

Ok, either I asked the question wrong or no one has the answer. Oh well…

Ok… well I guess I’ll have to close this thread. Is anyone not sure about what I mean at least?