You can quickly add a new message in Teamwork Projects no matter which part of your site that you're currently in, by using the 'Quick Add' button in the top right above the search bar:

You can click the 'Add Message' option, or to speed things up even more press 'Q' on your keyboard to bring up the menu then press 'N'.

When you choose to add a message in this menu from the dashboard or another screen outside any of your projects, you'll need to pick which project you want to add it to first, then you can use the new message creation window to post the message. If you're viewing a particular project at the time, the message will be added to that project by default but you can choose a different one using the 'Change Project' button