curl --request POST \
  --url https://api.example.com/api/v1/customer/bulk \
  --header 'Authorization: <api-key>' \
  --header 'Content-Type: application/json' \
  --data '
[
  {
    "externalId": "<string>",
    "externalBrandId": "<string>",
    "regDate": "<string>",
    "clickKey": "<string>",
    "externalProductId": "<string>",
    "username": "<string>",
    "country": "<string>",
    "country_code": "<string>"
  }
]
'

{
  "numImported": 123,
  "numDuplicates": 123,
  "numFailed": 123,
  "firstFailureId": "<string>",
  "firstFailureMessage": "<string>"
}