Press your name at the top right, then Turn on desktop notifications, and allow notifications when the browser asks. A notification then appears for each new message, also when FleetKraft is closed as long as the browser is running; click it to open the conversation.
Turn it on in each browser you use; logging out turns it off in that browser. If nothing appears, check that notifications for fleetkraft.com are allowed in the browser's site settings and in your computer's notification settings (including Focus or Do Not Disturb).