instead of trying a useless test post, I thought I use my first post to suggest adding a proper SSL certificate to the forum to make it also available via https. I do not know what server is used to host the forum, but if you need help setting this up, I am willing to help.
It is actually not that much I would assume. The site IS already available via SSL, it just presents the wrong certificate. If you have a Linux vServer, then installing one e.g. via the Lets Encrypt shell script (https://github.com/lukas2511/letsencrypt.sh) shouldn't be a big issue.