How to access a multi branch resource attribute in a concourse job?

Question

I'm using multi branch resourcing in a concourse pipeline like so:

resources:
- name: my-resource
  type: git-multibranch
  source:
    uri: git@github.com.../my-resource
    branches: 'feature/.*'
    private_key: ...  
    ignore-branches: ''

How can I access the branch the resource is on at the time the job runs? like so:

jobs:
...
    outputs:
  - name: my-resource
    params:
      GIT_BRANCH: {BRANCH-GOES-HERE}

I'm looking to access it via something like my-resource.branch but haven't found any thing that works yet