How to refer to the chosen map

Hi, another question today:

How can I can refer to the Map Window chosen board from another window ?
I mean, what’s the property that tells me which map was chosen from the initial player, from the dropdown menu shown at start ?
Would I need to look for a piece I know belong to that map, and call for example for $thatpiece$.CurrentBoard to know which board was chosen ?
Thanks for your help

Well that’s what I did.
With the Startup GKC, I run a key command at startup that calls a piece belonging to the map, at-start stack to board.
Then the $CurrentBoard$ sets the static value for the GP I’m interested in.
The piece don’t have an image nor a text. just the trait to set the GP …