If you go into the text editor of a dialog, a helpful popup appears that you can click to get the special tags like [PlayerName] or [OOC]. Unfortunately, it randomly doesn't go away and sticks around. Even if you go into map or mission preview mode.
There's a little X in that window to close it. Does that work?
Hehe... You mean sometimes people miss the obvious? Well, I did not. It didn't close, and it stuck around in Map Preview. Interestingly, the color scheme of the dialog changed in the map preview - I suppose the normal styles that are applied to it were either unavailable or change in preview mode.