Credit to Brady from the YNAB team. Assembled from here: https://support.youneedabudget.com/t/k9rxc9/using-google-apps-script-with-the-api

ynab_to_google_sheets.js

function get_ynab_categories(accessToken, budgetId) {
  const groups = fetch_ynab_data(accessToken, "budgets/" + budgetId + "/categories").category_groups;

  const columns = ["Name", "Budgeted", "Activity", "Balance"];
  const rows = [];

  for (var group_idx = 0; group_idx < groups.length; group_idx++) {
    // Add the group
    var group = groups[group_idx];
    // Skip internal and hidden categories
    if (['Internal Master Category', 'Hidden Categories'].indexOf(group.name) >= 0) continue;
    rows.push([group.name]);

    // Add the categories
    for (var category_idx = 0; category_idx < group.categories.length; category_idx++) {
      var category = group.categories[category_idx];
      var name = "      " + category.name; // Indent categories a bit so they are offset from groups
      // Calculate currency amounts from mulliunits
      var budgeted = category.budgeted / 1000.0;
      var activity = category.activity / 1000.0;
      var balance = category.balance / 1000.0;
      rows.push([name, budgeted, activity, balance]);
    }
  }

  return [columns].concat(rows);
}

function fetch_ynab_data(accessToken, path){
  const url = "https://api.youneedabudget.com/v1/" + path;
  const options = {
    "headers": {
      "Authorization": "Bearer " + accessToken
    }
  };
  const response = UrlFetchApp.fetch(url, options);
  const data = JSON.parse(response.getContentText()).data;
  return data;
}

function onOpen() {
  var sheet = SpreadsheetApp.getActiveSpreadsheet();
  var entries = [{
    name : "Refresh Categories",
    functionName : "refresh_ynab_categories"
  }];
  sheet.addMenu("YNAB", entries);
};

function refresh_ynab_categories() {
  SpreadsheetApp.getActiveSpreadsheet().getRange('E1').setValue(new Date().toTimeString());
}