How to map a variable associated to a trigger

How to map a variable associated to a trigger

Hello,
I would like to map the ID of a record associated to a trigger (when a task is updated, for example).
To do this ,
- I have builded a custom function associated to the module "Tasks".
- I use the getJson Deluge function on the argument "task" which is a map typ. 
- I configure a workflow rule which integrates the trigger and the custom function.





When I test, I manage to retrieve the content of the "task" argument.



But extracting the record ID using the getJson function doesn't work:



Any Help would be appreciated.