How to modify Active Tilemap / Valid Targets in Pellet Window?

I have been trying to figure out with custom brushes and editors in C# how to modify the Active Tilemap field seen highlighted in the picture.

113668-unity-question-1.png

The only reference I found in unity documentation seems to be in GridBrushEditorBase class with “GameObject validTargets” but that does not seem to work.
So any ideas on how to access this field would be very much appreciated.

did you ever figure this out? With my custom brush that area is grayed out. i have to select the grid in the hierarchy to paint on it which is annoying