index.html

<html>
  <head></head>
  <body>
    <script>
      // Register Service Worker
      if ('serviceWorker' in navigator) {
        window.addEventListener('load', () => {
          navigator.serviceWorker.register('/service-worker.js');
        });
      }
      function download() {
        // Get values from inputs
        const url = window.target_url.value;
        const filename = window.filename.value;

        const a = document.createElement('a');
        a.href = `/swdonwload?url=${encodeURIComponent(url)}&filename=${filename}`;
        a.target = '_blank';
        a.click();
      }
    </script>
    <input type="text" id="target_url" placeholder="URL" size="100"><br>
    <input type="text" id="filename" placeholder="File name"><br>
    <button onclick="download()">Download</button>
  </body>
</html>