Pass BusinessKey in expression


How do I access the Business Key in an expression?
I have a method in a Spring service, flowableBpmPlugin;
public void sendEmail(String reference)
System.out.println("sendEmail reference= "+reference);

I would like to pass in the business key from a service task. I have tried;


which passes null

In order to get this to work I have had to add a new method that accepts the execution.
public void sendEmail(DelegateExecution e)
String reference = e.getProcessInstanceBusinessKey();
System.out.println("sendEmail reference= "+reference);
and change the expression to


What do I need to do to pass the process instance business key as a String?