I've learned a lot from parsing through Andy Cotgreave workbooks, they demonstrate some lovely formatting techniques. You are correct in that reference lines are added to measures, in particular, the axes that measures create. The trick here is that the axis has been hidden, and once the axis is hidden there is only one way to edit the reference line is to find the reference line (which in this view has also been hidden), that can be done but there's an easier way: Unhide the axis.
You do that by right-clicking on the axis pill (or left-clicking on the drop-down area on the right-hand side of the pill) and choosing the Show Header option from the context menu:
Then the axis tick marks and title appear, you can right-click on the tick marks to edit the reference line, etc.
Alternatively, if you know a reference line is there, you can click on the reference line, which turns the (hidden) line red, then right-click on one of the lines to get the context menu where you can edit the reference line, like so:
Thanks very much, Jonathan! It works now~