const SiteClient = require('datocms-client').SiteClient;
const client = new SiteClient('YOUR-API-TOKEN');
client.whiteLabelSettings.update({
  customI18nMessagesTemplateUrl: 'https://my-app-messages.netlify.app/:locale/message.json'
})
.then((whiteLabelSettings) => {
  console.log(whiteLabelSettings);
})
.catch((error) => {
  console.error(error);
});

> node example.js
{
  "id": "312",
  "customI18nMessagesTemplateUrl": "https://my-app-messages.netlify.app/:locale/message.json"
}