next.js/examples/cloudflare-turnstile/pages/api/handler.ts

import type { NextApiRequest, NextApiResponse } from "next";

export default async function Handler(
  req: NextApiRequest,
  res: NextApiResponse,
) {
  const form = new URLSearchParams();
  form.append("secret", process.env.CLOUDFLARE_TURNSTILE_SECRET_KEY);
  form.append("response", req.body["cf-turnstile-response"]);
  form.append("remoteip", req.headers["x-forwarded-for"] as string);

  const result = await fetch(
    "https://challenges.cloudflare.com/turnstile/v0/siteverify",
    { method: "POST", body: form },
  );
  const json = await result.json();
  res.status(result.status).json(json);
}