mirage-config/examples/valueSubstitution/config.json

{
  "parameters": {
    "greeting": "Hello",
    "subject": "$CONFIG_EXAMPLE_SUBJECT",
    "followUp": "${CONFIG_EXAMPLE_FOLLOW_UP:Enjoy your day!}"
  },
  "start": "${parameters.greeting} ${parameters.subject}! ${parameters.followUp}",
  "end": "${parameters.farewell:Bye!}"
}