curl --request POST \
  --url https://api.kit.com/v4/tags/{tag_id}/subscribers/{id} \
  --header 'Content-Type: application/json' \
  --header 'X-Kit-Api-Key: <api-key>' \
  --data '{}'

{
  "subscriber": {
    "id": 123,
    "first_name": "<string>",
    "email_address": "<string>",
    "state": "active",
    "created_at": "<string>",
    "tagged_at": "<string>",
    "fields": {}
  }
}