It works also for me... maybe it depends on how Michael Wood is passing the json object to the method... 

If he is passing a string, there is a conversion needed from string to %DynamicAbstractObject like:

Set str="{""prio"":""2"",""code"":""A001""}"
Write str
{"prio":"2","code":"A001"}
Set obj=##Class(%DynamicAbstractObject).%FromJSON(str)
Write obj
1@%Library.DynamicObject
ZW obj
obj={"prio":"2","code":"A001"}  ; <DYNAMIC OBJECT>

Hi, in order to do that you need to create a resource first (see: https://docs.intersystems.com/irisforhealthlatest/csp/docbook/DocBook.UI...), assign that resource to your application (see: https://docs.intersystems.com/irisforhealthlatest/csp/docbook/DocBook.UI...) and or zen page (see: https://docs.intersystems.com/ens201817/csp/docbook/Doc.View.cls?KEY=GZA... ) then assign this resource to your role (see: https://docs.intersystems.com/irisforhealthlatest/csp/docbook/DocBook.UI...) and then assign this role to your users.

Hope this can help

Those are system routines so you can not find their source code available.

But you do not need to import them into your project because the compiled version is always available in the destination environment where you plan to import the project.  

As Eduard Lebedyuk say is better to use the new $SYSTEM functions.