test.html

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Carbone Docker wrapper tester</title>
</head>
<body>
  <form onsubmit="formatData()" action="/render" method="post" target="_blank" enctype="multipart/form-data">

    <input type="file" name="template" id="template">
    <input type="hidden" name="data" id="data">
    <input type="hidden" name="format" value="pdf">
    <input type="hidden" name="outputName" value="test.pdf">

    <textarea name="_data" id="_data" cols="30" rows="10">{"firstname":"John"}</textarea>

    <input type="submit" value="Générer">

  </form>
  <script>
    function formatData() {
      const _data = document.getElementById(`_data`).value;
      let data = {};

      try {
        data = JSON.parse(_data);
      } catch (e) {
        console.error(`Cannot parse data : ${e}`);
      }

      document.getElementById(`data`).value = JSON.stringify(data);
    }
  </script>
</body>
</html>