See this demo:
dropbox.com/scl/fi/8eqh8e5ryrszk1x6aflwq/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)