We can use https. However, what I discovered when I investigated the subscription page a while ago, it won't work with a Referer of 127.0.0.1. I expect it's the same thing here, which means there is nothing we can do about it.
(Which also means, this is NOT your cue to head over to the Bug Report forum and open a Bug Report - unless you really want us to close it immediately with a "Will Not Fix")