"Download OS" should let you paste a URL to an iso.
I should be able to paste a URL like this: https://releases.ubuntu.com/19.10/ubuntu-19.10-desktop-amd64.iso
Into "Download OS", there should also be an explicit UI to do the same thing.
Ideally, if you paste in a web page and that has just one link to an ISO, then that should do the same thing, e.g. the download page for the above iso: