NT has a number of builtin chart markers that you can use for this purpose without having to go through the hassle of building your own. There are up/down arrows, up/down triangles, etc.
These reside under Drawing Tools > Chart Markers and they all have keyboard shortcuts so you can quickly place one on your chart.
I’m not trying to discourage you from building your own custom drawing tools if that’s your intention. Just saying there’s a free existing alternative that can be used if all you want to do is manually mark turning points on your chart.