const SiteClient = require('datocms-client').SiteClient;
const client = new SiteClient('YOUR-API-TOKEN');
const modelIdOrApiKey = 'blog_post';
client.fieldset.create(modelIdOrApiKey, {
  title: 'SEO-related fields',
  hint: 'Please fill in these fields!',
  position: 1,
  collapsible: true,
  startCollapsed: false
})
.then((fieldset) => {
  console.log(fieldset);
})
.catch((error) => {
  console.error(error);
});

> node example.js
{
  "id": "124",
  "title": "SEO-related fields",
  "hint": "Please fill in these fields!",
  "collapsible": true,
  "startCollapsed": false,
  "position": 1,
  "itemType": "44"
}