curl --request POST \
  --url https://api.kit.com/v4/bulk/custom_fields \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '{
  "custom_fields": [
    {
      "label": "Test Custom Field 0"
    },
    {
      "label": "Test Custom Field 1"
    },
    {
      "label": "Test Custom Field 2"
    },
    {
      "label": "Test Custom Field 3"
    }
  ],
  "callback_url": null
}'

{
  "custom_fields": [
    {
      "id": 87,
      "label": "Existing Custom Field",
      "key": "existing_custom_field",
      "name": "ck_field_87_existing_custom_field",
      "created_at": "2023-02-17T11:43:55Z"
    },
    {
      "id": 89,
      "label": "Interests",
      "key": "interests",
      "name": "ck_field_89_interests",
      "created_at": "2023-02-17T11:43:55Z"
    },
    {
      "id": 88,
      "label": "Last name",
      "key": "last_name",
      "name": "ck_field_88_last_name",
      "created_at": "2023-02-17T11:43:55Z"
    }
  ],
  "failures": []
}