IAP Google Cloud CORS Options

I have followed the documentation https://cloud.google.com/iap/docs/customizing and created a file 'settings'

access_settings:
  cors_settings:
    allow_http_options: "true"

I have resource behind IAP under: All Web Services/Backend services. It is called foo.

I have selected Enable HTTP Options in console for that resource and also executed following command:

gcloud iap settings set settings --project=PROJECT_ID

but I still receive "No 'Access-Control-Allow-Origin' header is present on the requested resource."

What am I doing wrong?