You can't when using the normal Tableau legend, but that's a good Idea. You post it I'll vote for it. Meantime you can create a custom legend in a graphics program and then display it as an image. The downside is highlight actions won't work.
What I do I I add it as a shape on the bar graph and then it appears on the legend. Drag to rows/columns. Dual Axis. Change to Shape. Pick shape to indicate reference line (or custom shapes). Now it appears in legend.