Not even for signing up with your email and a password.
I would suggest you contact Piriform directly about this
Im not sure if it went through, I received no email and it took me to a blank page when I hit submit.
It's not just the forum, it's site-wide, many pages on Piriform.com even if you initially open them using HTTPS will revert to non-secure when clicking a link. For example the Docs site https://www.piriform.com/docs where I'd much prefer to post a link to it in HTTPS which it will load fine as secure, but clicking any links shown on the page will make it non-secure.