Whilst you do have the ability to create new threads within Basecamp, we would advise that you post your message within one of the existing threads first. This will allow us to keep your Basecamp project structured.
If there's something that we think requires a new thread, such as a new sub-project or a new channel that we're going to run campaigns on, then we'll set up the new thread with a clear first message as to why we've set up a thread.