I was initially having difficulty right clicking a specific layer so that I could add G-code to it, when I right clicked, the dot on the layer selection bar would shift to the wrong layer.
I figured out a way to make it easy to select the specific layer. If you right click on the layer selection bar and the pop up box with the “add custom G-code” is visible, you can then click the up and down arrows on the keyboard and go to the layer you want, then you click the “add custom G-code” option of the pop-up box and it will add it to the layer you selected with the up/down arrows.