const options = {
  method: 'POST',
  headers: {Authorization: 'Bearer <token>', 'Content-Type': 'application/json'},
  body: '{"front_image_url":"https://s3.amazonaws.com/cdn.thanks.io/user_image_library/gFBaJJJWRp60op3ZOoK7UaO5I7zi33rdKQGgHylR.png","handwriting_style":5,"message":"This is a test message","preview":true,"recipients":[{"name":"Current Resident","address":"123 Main Street","city":"Fake City","province":"NY","postal_code":"55555","country":"US"}]}'
};

fetch('https://api.thanks.io/api/v2/send/windowlessletter', options)
  .then(response => response.json())
  .then(response => console.log(response))
  .catch(err => console.error(err));

{
  "success": true,
  "code": 200,
  "message": "OK",
  "data": {
    "previews": [
      "https://s3.amazonaws.com/content.thanks.io/sls-handwriting-engine/images/59be0dca-e2dc-8d90-ea24-8d4b29cbc7c1.png"
    ]
  }
}