Hi ​@suminchoi2015 - Thanks for you questions. 

This happens because Klaviyo’s CSV importer treats all non-standard fields as custom properties by default.

The external_id field is a reserved API field that can only be populated via the Profiles API, not via CSV upload.

When you upload a CSV file, Klaviyo doesn’t interpret external_id as a special field, it just sees it as a custom attribute and stores it under “Custom properties.”

Solution

To have the external_id appear under Profile Details, you must set it using the Profiles API.

Here’s the proper way to do it:

POST https://a.klaviyo.com/api/profiles/
Authorization: Klaviyo-API-Key <PRIVATE_API_KEY>
Content-Type: application/json

{
  "data": {
    "type": "profile",
    "attributes": {
      "email": "test@test.com",
      "external_id": "1234567891",
      "first_name": "Test",
      "last_name": "User"
    }
  }
}