Prefill/update WordPress user profile field from other form

I've added a custom user profile field in a WordPress video hosting theme, and I'd like to make that field visible and prefilled (if it's previously been filled out) on the video submission page. Of course, the link needs to be two-way so that if it's blank, and the user enters new information in that field, their profile is updated. I know exactly what file needs to be edited in the theme, and even where I want it edited, but I can't (easily) work out how to update the field. So if this is something you'd like to tackle, I look forward to working with you. Note: In my experience auto-bidders tend to waste my time, so to weed them out, please mention me by name (Tim) in your bid to be considered. Thank you!