How to extract data as a plain string


None of the examples I found tell me how to extract information from input as a plain string. It seems like it should be trivial. For the following model:

  "invocation": "create task",
  "version": "4.0",
  "intents": {
    "createTaskIntent": {
      "phrases": ["create task {task}"]

using any NLU or no NLU, how would I extract the text that was matched for {task}?