curl --request POST \
  --url https://app.getcensus.com/api/v1/sources/{source_id}/filter_segments \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '{
  "name": "priority accounts",
  "molecules": [
    {
      "attribute": "last_seen_at",
      "attribute_type": "property",
      "operator": "isnotnull",
      "value": "id"
    }
  ],
  "dataset_id": 123
}'

{
  "status": "created",
  "data": {
    "id": 9
  },
  "filter_segment_source_url": "https://app.getcensus.com/api/v1/sources/12/filter_segments/9/source_status"
}