Mattermost
This lecture uses Mattermost instead of a forum other lectures might use. Use the following link to sign up: https://cms-mattermost.cispa.de/signup_user_complete/?id=gt3oafcefb8y9ex9ubhsftbwso
While you can use Mattermost in the browser there also exist installable applications for various operating systems (Windows, Mac, Linux, IOS, Android etc.).
You may want to use the following link to add the server: https://cms-mattermost.cispa.de
(there also exists a meme channel)