You have to log into WUnderground and create the device first. You set it up as a URL camera, but leave it blank. Once that is complete, it will give you a Camera ID.
Then you login to the WUN.bloomsky.com site with your Bloomsky ID. Fill out the form with that camera ID. The Bloomsky does the uploads for you.
It's been a few years for me, but that's what I recall...