curl -X PATCH "https://api.fitsociety.io/public/v1/clients/{clientId}/custom-fields" \
  -H "Authorization: Bearer <access_token>" \
  -H "Idempotency-Key: <stable_request_key>" \
  -H "Content-Type: application/json" \
  -d '{}'

const options = {
  method: 'PATCH',
  headers: {
    'Idempotency-Key': '<idempotency-key>',
    Authorization: 'Bearer <token>',
    'Content-Type': 'application/json'
  },
  body: JSON.stringify({customFieldValues: [{key: 'shirt_size', value: '<string>'}]})
};

fetch('https://api.fitsociety.io/public/v1/clients/{clientId}/custom-fields', options)
  .then(res => res.json())
  .then(res => console.log(res))
  .catch(err => console.error(err));

import requests

url = "https://api.fitsociety.io/public/v1/clients/{clientId}/custom-fields"

payload = { "customFieldValues": [
        {
            "key": "shirt_size",
            "value": "<string>"
        }
    ] }
headers = {
    "Idempotency-Key": "<idempotency-key>",
    "Authorization": "Bearer <token>",
    "Content-Type": "application/json"
}

response = requests.patch(url, json=payload, headers=headers)

print(response.text)

{
  "data": {
    "clientId": "<string>",
    "customFields": [
      {
        "key": "<string>",
        "label": {
          "en": "<string>",
          "nl": "<string>"
        },
        "type": "<string>",
        "value": "<string>",
        "updatedAt": "2023-11-07T05:31:56Z"
      }
    ]
  },
  "meta": {
    "requestId": "4f849d7d-f4f1-45cc-b4b7-3984a3d17f83",
    "rateLimit": {
      "limit": 10,
      "remaining": 9,
      "resetSeconds": 1,
      "retryAfterSeconds": 1
    },
    "idempotency": {
      "replayed": true
    }
  }
}