Help with posting on existing threads

Hello

I'm relatively new to the chat and have been enjoying reading the various forums and threads. One thing I can't figure out though is how to post on a thread without replying directly to someone else's post. Can anyone help me with this? There doesn't appear to be any buttons available anywhere, other than the Reply buttons on other people's posts and the button to start a new discussion. Have tried both on mobile and laptop. 

I also noticed the counts on some threads which includes the number of "subscribers" - how do you subscribe to a thread? 

Many thanks Blush