The correct place would have been in the General forum - ‘anything and everything about this board.’ Though it is also given some latitude to post off-topic (unrelated to avast). The my.avast.com forum is relating to that web URL - basically questions relating to your avast account and that site.
I don’t know why or when this changed (possibly on the relaunch of the avast forum), but previously this was the default option to receive notifications to your topics/posts.
If you haven’t already found this, try changing your forum Profile - Modify Profile > Notifications - enable the ‘Turn notification on when you post or reply to a topic.’ Note previously there was a post count restriction on what elements of the profile settings you could change - I don’t know if that is still in force or if this option would be accessible.