See this demo:
dropbox.com/s/h8juh80jq4d93fi/Minimap.capx
If you want to show a small selection box on the minimap, you can set its position to
clamp(Mouse.X("Minimap"), SelectionBox.width/2, Minimap.BBoxRight-SelectionBox.width/2)
clamp(Mouse.Y("Minimap"), SelectionBox.height/2, Minimap.BBoxBottom-SelectionBox.height/2)