vercel__next.js/examples/cms-enterspeed/pages/api/preview.js

export default async function preview(req, res) {
  // Check the secret and next parameters
  // This secret should only be known by this API route
  if (req.query.secret === undefined || null) {
    return res.status(401).json({ message: "No token provided" });
  }

  if (req.query.secret !== process.env.ENTERSPEED_PREVIEW_SECRET) {
    return res.status(401).json({ message: "Invalid token" });
  }

  res.setDraftMode({ enable: true });

  res.redirect("/");
}