Install alltray by entering the following commands in the terminal:
sudo apt update sudo apt install alltray
Dock any program into the system tray
With AllTray you can dock any application with no native tray icon (like Evolution, Thunderbird, Terminals) into the system tray. A high-light feature is that a click on the "close" button will minimize back to system tray. It works well with GNOME, KDE, Xfce 4*, Fluxbox* and WindowMaker*. (*) No drag 'n drop support. Enable with "-nm" option.