I think it is one of the number keys. https://forum.studio-397.com/index.php?threads/default-keybindings.51526/
try assigning a button to MESSAGE HISTORY. I'm not 100% sure this is the best method, but it should display that window again.
Also It's in player.JSON: "Message Center Detail":3, Note that it disappears after a time, pops up again when there is a new message