To message a specific reservation, you can access the message window in two different places.
From the Reservation panel
The first one is on the reservation panel:
Select the card for the party you want to message, on the lower right-hand side, you see the button "Show Chat"
On the Table Management panel
The second place you can access it is on the Table Management panel.
Go to the reservation tab on the lower right-hand side.
Select the reservation you want to send a message to.
Click on the "Show Chat" button on the lower right end side.
You can now communicate with this guest.