I doubt you can actually access this property. It is at the very least undocumented.
You will either have to use boxplot or build your own version (or use of the many from the file exchange).
When Mathworks introduced this I was surprised. It is nice to have a boxplot function that doesn't require a toolbox, but why they would want to support two separate functions is a mystery to me. Maybe they want to still push sales of the toolbox for people who want to set the Whisker property but don't want to write their own function?
