Hi Paul,
Unfortunately, the ability to pass uri is not currently available via the html subscription form, it does have to be the url with the feed id included.
It is a good idea to provide that functionality for folks who want to customize the form for multiple feeds.
Also, one thing to note is that you need activate the email service for each of the feeds you want to offer email subscription for. Even if you offer the form (with a valid feed id) on your site to users, the service needs to be activated via
feedburner.com for that id or it will not work.
Hope this helps.