curl --request PUT \
  --url https://api.kit.com/v4/subscribers/{id} \
  --header 'Content-Type: application/json' \
  --header 'X-Kit-Api-Key: <api-key>' \
  --data '
{
  "first_name": "Alice",
  "email_address": "alice@convertkit.dev",
  "fields": {
    "last_name": "Lamarr",
    "birthday": "Feb 17",
    "source": "landing page",
    "role": "Software developer",
    "company": "Convertkit",
    "postal_code": "83702",
    "website": "convertkit.com",
    "social_media": "https://www.linkedin.com/company/convertkit",
    "how_did_you_hear_about_us": "Social media",
    "interests": "Monetization",
    "coupon": ""
  }
}
'

{
  "subscriber": {
    "id": 378,
    "first_name": "Alice",
    "email_address": "alice@convertkit.dev",
    "state": "active",
    "created_at": "2023-02-17T11:43:55Z",
    "fields": {
      "birthday": "Feb 17",
      "last_name": "Lamarr",
      "source": "landing page"
    }
  }
}