Field Injection for Output

Hello there,

I successfully use Field Injection into an ExecutionListener for same tasks. I can read and write back to injected expression. (write back in case of L-Value expression, like below)

<camunda:executionListener class="" event="start">
  <camunda:field name="myField">

But how can I create Output-Only expression, that creates me a new process instance variable. The goal to achieve is, that process definition defines the name of the process instance variable and not the ExecutionListener does. Listener uses injected expression to write the data without knowning the name of variable.

This does not work, but generate no exeption but the newly created expected variable is null.

  <camunda:field name="myField">

What is the correct way to achieve it?