Unable to set default values

[laciferin2024]

local getExt(varName, default) = 
  if std.extVar(varName) != null then std.extVar(varName) else default;

{
  machine: {
    gpu: 0,
    cpu: 100,
    ram: 100,
  },
  job: {
    APIVersion: "V1beta1",
    Spec: {
      Deal: {
        Concurrency: 1,
      },
      Docker: {
        Entrypoint: [
          "/usr/games/cowsay",
          getExt("Message", "Pass me an input called Message, like darts run cowsay:v1.0.0 -i Message=moo")
        ],
        Image: "ghcr.io/darts2024/cowsay:v1.0.0",
      },
      Engine: "Docker",
      Network: {
        Type: "None",
      },
      PublisherSpec: {
        Type: "local",
      },
      Resources: {
        GPU: "",
      },
      Timeout: 500,
      Verifier: "Noop",
    },
  },
}

###Error

jsonnet default.dart 
RUNTIME ERROR: Undefined external variable: Message
        default.dart:2:6-25     function <getExt>
        default.dart:19:11-108  thunk from <object <anonymous>>
        Array element 1
        Field "Entrypoint"
        Field "Docker"
        Field "Spec"
        Field "job"
        During manifestation

Version:

Jsonnet commandline interpreter (Go implementation) v0.20.0