accessing user defined pipeline variables within definitions steps bitbucket

Im trying pass a user defined variable from a custom pipeline to a step defined within the definitions section.

My yml snippet is below:

definitions:
  steps:
    - step: &TagVersion
        trigger: manual
        script:
          - export VERSION=$VERSION
          - echo "VERSION $VERSION"

  custom:
    run_custom:
      - variables:
          - name: VERSION
      - step:
          script:
            - echo "starting"
      - parallel:
          - step: 
              <<: *TagVersion
              variables:
                VERSION: $VERSION

When I build the pipeline I can see the variable is listed as a pipeline variable when running the step TagVersion, and the correct value is shown there but dont know how to use this within the scripts section where im trying to echo out the value.

thanks