curl --request POST \
  --url https://{organization}.rescueconsole.com/api/people/custom-fields \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: application/json' \
  --data '
{
  "field_key": "tshirt_size",
  "field_label": "T-shirt size",
  "field_type": "select",
  "options": [
    "S",
    "M",
    "L",
    "XL"
  ],
  "required": false,
  "sort_order": 10
}
'

const options = {
  method: 'POST',
  headers: {Authorization: 'Bearer <token>', 'Content-Type': 'application/json'},
  body: JSON.stringify({
    field_key: 'tshirt_size',
    field_label: 'T-shirt size',
    field_type: 'select',
    options: ['S', 'M', 'L', 'XL'],
    required: false,
    sort_order: 10
  })
};

fetch('https://{organization}.rescueconsole.com/api/people/custom-fields', options)
  .then(res => res.json())
  .then(res => console.log(res))
  .catch(err => console.error(err));

import requests

url = "https://{organization}.rescueconsole.com/api/people/custom-fields"

payload = {
    "field_key": "tshirt_size",
    "field_label": "T-shirt size",
    "field_type": "select",
    "options": ["S", "M", "L", "XL"],
    "required": False,
    "sort_order": 10
}
headers = {
    "Authorization": "Bearer <token>",
    "Content-Type": "application/json"
}

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

print(response.text)

{
  "id": "<string>",
  "entity_type": "person",
  "field_key": "<string>",
  "field_label": "<string>",
  "field_type": "text",
  "options": [
    "<string>"
  ],
  "required": true,
  "sort_order": 123,
  "builtin": true,
  "section": "<string>",
  "default_roles": [
    "adopter"
  ],
  "note": "<string>"
}

{
  "error": "house_trained must be 1, 0 or null (not assessed)"
}

{
  "error": "house_trained must be 1, 0 or null (not assessed)"
}

{
  "error": "house_trained must be 1, 0 or null (not assessed)"
}

{
  "error": "house_trained must be 1, 0 or null (not assessed)"
}