Seeing as this thread is in regards to the Forums, it has been moved from the Bug Report section.
As @fizl101 stated, there is no payment required to use any function of the ESO Forums, and the available Reactions members can leave on posts are Insightful, Agree, or Awesome. If on PC, the reaction buttons will appear when hovering over a post.
I hope this has cleared things up for you, @Neugeniko.
To leave a comment or reply on a forum thread, simply scroll to the very bottom and you should see a section called "Leave a Reply" where you can type a comment and then post with the "Post Reply" button.