Display custom layer by default with URL?

I've created a map I want to share with others where the URL contains the ll parameter to center the map on a set of coordinates, z to set the zoom level and b for the base layer map. These all work perfectly. I have a single custom layer that I would like to display "on" by default when the map is loaded by URL. How does one show the map with the custom layer displayed by default? I have checked the "Update this map's default display to use the currently selected set of layers." checkbox on the map info dialog, but it doesn't seem to work.