Proxying the script

<script
  defer
  data-domain="example.com"
  data-api="/stats/api/event"
  src="/stats/js/sift.js"
></script>

export default {
  async fetch(request) {
    const url = new URL(request.url);

    if (url.pathname === "/stats/js/sift.js") {
      const upstream = await fetch("https://sift.agentonboard.xyz/js/sift.js", {
        cf: { cacheTtl: 86400 },
      });
      return new Response(upstream.body, {
        headers: {
          "content-type": "application/javascript; charset=utf-8",
          "cache-control": "public, max-age=86400",
        },
      });
    }

    if (url.pathname === "/stats/api/event" && request.method === "POST") {
      // Forward the visitor's identity inputs. Without them every visitor
      // looks like the proxy.
      const headers = new Headers({ "content-type": "text/plain" });
      const ip = request.headers.get("cf-connecting-ip");
      const ua = request.headers.get("user-agent");
      if (ip) headers.set("cf-connecting-ip", ip);
      if (ua) headers.set("user-agent", ua);

      return fetch("https://sift.agentonboard.xyz/api/event", {
        method: "POST",
        body: request.body,
        headers,
      });
    }

    return new Response("Not found", { status: 404 });
  },
};

const nextConfig = {
  async rewrites() {
    return [
      { source: "/stats/js/sift.js", destination: "https://sift.agentonboard.xyz/js/sift.js" },
      { source: "/stats/api/event", destination: "https://sift.agentonboard.xyz/api/event" },
    ];
  },
};

location = /stats/js/sift.js {
    proxy_pass https://sift.agentonboard.xyz/js/sift.js;
    proxy_set_header Host sift.agentonboard.xyz;
}

location = /stats/api/event {
    proxy_pass https://sift.agentonboard.xyz/api/event;
    proxy_set_header Host sift.agentonboard.xyz;
    proxy_set_header Content-Type text/plain;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
}