I tried this with $${{ secrets.FOO }} and it didn't work

Not all variables which may depend on an environment are available at every level of your workflows. Within the scope of a job you'll have secrets available but that may not be the case for configuration at a level above jobs. (Or maybe it's the double dollar notation.) So without linking the full workflow it's hard to say what a solution might be @PaulRBerg

default: ${{ github.event.repository.name }}

Doesn't work either (in workflow_dispatch at least). Fails with:

The workflow is not valid. .github/workflows/myworkflow.yml (Line: 27, Col: 18): Unrecognized named-value: 'github'. Located at position 1 within expression: github.event.repository.name

I resorted to no default value and then adding an additional step with bash parameter expansion and step outputs as suggested above. However, set-output is deprecated so it looks different now. This is a ton of boiler plate code to do what should be the one-liner default value:

    steps:
      -
        name: Sort out the input param and set default if needed
        id: getlabel
        run: |
          LABEL_INPUT=${{ github.event.inputs.label }}
          echo "label=${LABEL_INPUT:-myorg/${{ github.event.repository.name }}}" >> $GITHUB_OUTPUT
      -
        echo ${{ steps.getlabel.outputs.label }}