curl --request POST \
  --url https://api.kit.com/v4/forms/{form_id}/subscribers/{id} \
  --header 'Content-Type: application/json' \
  --header 'X-Kit-Api-Key: <api-key>' \
  --data '
{
  "referrer": "https://mywebsite.com/bfpromo/?utm_source=facebook&utm_medium=cpc&utm_campaign=black_friday&utm_term=car_owners&utm_content=get_10_off"
}
'

{
  "subscriber": {
    "id": 731,
    "first_name": "Alice",
    "email_address": "alice@convertkit.dev",
    "state": "active",
    "created_at": "2023-02-17T11:43:55Z",
    "added_at": "2023-02-17T11:43:55Z",
    "fields": {},
    "referrer": "https://mywebsite.com/bfpromo/?utm_source=facebook&utm_medium=cpc&utm_campaign=black_friday&utm_term=car_owners&utm_content=get_10_off",
    "referrer_utm_parameters": {
      "source": "facebook",
      "medium": "cpc",
      "campaign": "black_friday",
      "term": "car_owners",
      "content": "get_10_off"
    }
  }
}