Take a look at this post from David Chartier, entitled macOS: How to send iMessages without the Messages app open.
The post itself is interesting, worth a read even if you have no interest in sending a message in this sidebar approach. The value is in learning about widgets (if they are new to you) and, specifically, the Social widget. Lots of nuance here:
- Two finger drag from the right side of the trackpad to bring up the Notifications/Today sidebar.
- Look through the widgets, learn to add/delete them from the list.
- Play with the Social widget, click the info button to customize.
Good stuff.