curl --request POST \
  --url https://app.herondata.io/api/end_user_entities/{entity_heron_id}/attributes/{attribute_heron_id}/sources \
  --header 'Content-Type: application/json' \
  --header 'x-api-key: <api-key>' \
  --data '{
  "sort_order": 123,
  "source_config": {},
  "source_type": "parsed_end_user_file"
}'

{
  "heron_id": "<string>",
  "sort_order": 123,
  "source_config": {},
  "source_type": "parsed_end_user_file"
}