Can you push to a pull request branch in Concourse?

In Concourse we are using teliaoss/github-pr-resource to run pull request checks when a pull request has been created in Github. One of the checks we do is npm run prettier:fix which ensures all code is formatted to standards. If the repository shows changes then the task fails and the user has to run the command locally and push changes. This is fine, but we'd love to run npm run prettier:fix and then commit the changes to the pull request branch and avoid requiring the user to make another commit.

The git resource is available and allows you to push to a repository, but you have to specify the branch in your yaml, I'm not aware of a way to make that dynamic. Here's a simplified example using the git resource

resource_types:
  - name: pull-request
    type: registry-image
    source:
      repository: teliaoss/github-pr-resource

resources:
- name: deployment-plan
  type: git
  source:
    branch: main # this needs to be the pull-request branch
    uri: ((git-base-uri))/myproject

- task: run-prettier
  config:
    platform: linux
    image_resource:
      type: registry-image
      source:
        repository: alpine/git
    inputs:
      - name: deployment-plan
    outputs:
      - name: deployment-plan-git-update
    run:
      path: sh
      args:
        - -exc
        - |
          git clone deployment-plan deployment-plan-git-update
          cd deployment-plan-git-update
          npm run prettier:fix
          git add .
          if [[ ! -z "$(git status --porcelain)" ]]; then
            git commit -m "run prettier"
          fi
  - put: deployment-plan
    params:
      repository: deployment-plan-git-update
    rebase: true

You still need to explicitly set your branch, but the answer from @Busches got me on the right path

- task: prettier
  config:
    platform: linux
    image_resource:
      type: docker-image
      source:
        repository: image-with-node-git-and-jq
        tag: 14
    inputs:
    - name: pull-request
    outputs:
      - name: pull-request
    params:
      USERNAME: ((access-token.username))
      ACCESS_TOKEN: ((access-token.git-access-token))
    run:
      path: sh
      dir: pull-request
      args:
      - -exc
      - |
        export NG_CLI_ANALYTICS=false

        prNumber=$(cat .git/resource/metadata.json | jq -r '.[] | select(.name=="pr") | .value')
        branchName=$(curl 'https://'${USERNAME}:${ACCESS_TOKEN}'@github.nwie.net/api/v3/repos/Nationwide/roundup/pulls/'${prNumber} | jq -r '.head.ref')

        git fetch
        git checkout ${branchName}

        # run any commands that change files here
        npm install -g prettier
        npm run prettier:fix

        git add .

        # only commit if there are changes otherwise the step fails
        if [[ ! -z "$(git status --porcelain)" ]]; then
          git commit -m "prettier fix"

          git push --set-upstream origin ${branchName}
        else
          echo "no changes found"
        fi