curl --request PATCH \
  --url https://api.railmail.app/api/v1/custom-fields/{key}/config \
  --header 'Content-Type: application/json' \
  --header 'X-API-Key: <api-key>' \
  --data '
{
  "options": [
    {
      "value": "<string>",
      "label": "<string>"
    }
  ],
  "defaultValue": "<string>"
}
'

{
  "id": "3c90c3cc-0d44-4b50-8888-8dd25736052a",
  "key": "<string>",
  "displayName": "<string>",
  "description": "<string>",
  "type": "<string>",
  "status": "<string>",
  "isSystem": true,
  "isRequired": true,
  "defaultValue": "<string>",
  "createdAt": "2023-11-07T05:31:56Z",
  "updatedAt": "2023-11-07T05:31:56Z",
  "archivedAt": "2023-11-07T05:31:56Z"
}