curl --request POST \
  --url https://api.kit.com/v4/bulk/custom_fields/subscribers \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "custom_field_values": [
    {
      "subscriber_id": 0,
      "subscriber_custom_field_id": 0,
      "value": "value_0"
    },
    {
      "subscriber_id": 1,
      "subscriber_custom_field_id": 1,
      "value": "value_1"
    },
    {
      "subscriber_id": 2,
      "subscriber_custom_field_id": 2,
      "value": "value_2"
    },
    {
      "subscriber_id": 3,
      "subscriber_custom_field_id": 3,
      "value": "value_3"
    }
  ],
  "callback_url": null
}
'

{
  "custom_field_values": [
    {
      "subscriber_id": 525,
      "subscriber_custom_field_id": 102,
      "value": "Smith"
    },
    {
      "subscriber_id": 525,
      "subscriber_custom_field_id": 103,
      "value": "Acme Inc"
    }
  ],
  "failures": [
    {
      "errors": [
        "Subscriber does not exist"
      ],
      "custom_field_value": {
        "subscriber_id": null,
        "subscriber_custom_field_id": 102,
        "value": "Jones"
      }
    },
    {
      "errors": [
        "Custom field does not exist"
      ],
      "custom_field_value": {
        "subscriber_id": 525,
        "subscriber_custom_field_id": 999999,
        "value": "Test"
      }
    }
  ]
}