curl --request POST \
  --url https://api.hubapi.com/automation/v4/actions/callbacks/complete \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '{
  "inputs": [
    {
      "outputFields": {},
      "callbackId": "<string>"
    }
  ]
}'

This response has no body data.