mouseDown = false
startMouseX = 0
startMouseY = 0

 function GM:GUIMousePressed(mc)
  if (!mouseDown) then
    startMouseX,startMouseY = gui.GetMousePos() -- whatever
    mouseDown = true
 end
end
function GM:DrawRectang()
 if(mouseDown) then
 draw.RoundedBox( 5, startMouseX, startMouseY, 15, 15, Color(0, 255, 0, 150))
 end
end
 hook.Add( "HUDPaint", "mousepressed", DrawRectang )