curl --request POST \
  --url https://api.kit.com/v4/bulk/tags/subscribers \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '{
  "taggings": [
    {
      "tag_id": 0,
      "subscriber_id": 0
    },
    {
      "tag_id": 1,
      "subscriber_id": 1
    },
    {
      "tag_id": 2,
      "subscriber_id": 2
    },
    {
      "tag_id": 3,
      "subscriber_id": 3
    }
  ],
  "callback_url": null
}'

{
  "subscribers": [
    {
      "id": 541,
      "first_name": "Sub",
      "email_address": "[email protected]",
      "created_at": "2023-02-17T11:43:55Z",
      "tagged_at": "2023-02-17T11:43:55Z"
    },
    {
      "id": 541,
      "first_name": "Sub",
      "email_address": "[email protected]",
      "created_at": "2023-02-17T11:43:55Z",
      "tagged_at": "2023-02-17T11:43:55Z"
    }
  ],
  "failures": [
    {
      "errors": [
        "Subscriber does not exist"
      ],
      "tagging": {
        "tag_id": 74,
        "subscriber_id": null
      }
    },
    {
      "errors": [
        "Tag does not exist"
      ],
      "tagging": {
        "tag_id": null,
        "subscriber_id": 541
      }
    }
  ]
}