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": "[email protected]",
  "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": 309,
    "first_name": "Alice",
    "email_address": "[email protected]",
    "state": "active",
    "created_at": "2023-02-17T11:43:55Z",
    "fields": {}
  }
}