mouseDown = false
startMouseX = 0
startMouseY = 0

 function GUIMousePressed(mc)
  if (!mouseDown) then
    startMouseX,startMouseY = gui.MousePos() -- whatever
    mouseDown = true
 end
end
function DrawRectang()

	local mpos = gui.MouseX()
    local ypos = gui.MouseY()
 if (mousedown) then
 draw.RoundedBox( 5, startMouseX, startMouseY, mpos, ypos, Color(0, 255, 0, 150))
 //draw.RoundedBox( 5, startMouseX, startMouseY, 15, 15, Color(0, 255, 0, 150))
 end
end