A work around for a horizontal line is to use the "HorizontalLine" command at the end of Hotkeys script above and a line will be drawn. Holding down the left mouse you can then move the line around to pick best fit for the alert. Let go of the left mouse button the arrow next to the "A" comes up to set the alert price. It won't let you click on top of the horizontal line you've drawn on the chart but go to price axis and click where the horizontal line has the box with a cross in it.
This is a clunky method but at least you can see where your alert is.
Right click the horizontal line and you can config colour/line as you want it.