form.html

<form id="form" action="https://ihdyhw2gui.execute-api.us-east-1.amazonaws.com/stage?operation=" Accept="image/*" Content-Type="application/json" method="POST">
  Operation: <input type="text" name="operation" id="operation"><br>
  Options: <input type="text" name="options"><br>
  Your image: <input type="file" name="pic" id="pic" onchange="getBase64()"><br>
  <input type="hidden" name="base64Image" id="base64Image" value="">
  <img src="" height="200" alt="Image preview..." style="display:none">
  <input type="submit" value="Submit">
</form>

<script>

  var operation = document.getElementById("operation");
  var form_el = document.getElementById("form");
  var base64Image = document.getElementById("base64Image");
  var preview = document.querySelector('img');
  var pic = null;

  function getBase64() {
    pic = document.getElementById("pic");
    var reader = new FileReader();
    var file = pic.files[0];
    if (file) {
      reader.readAsDataURL(file);
    }
    reader.onload = function () {
      base64Image.value = reader.result.replace("data:image/png;base64,", "");
      preview.src = reader.result;
      preview.style.display = 'block';
    };
    reader.onerror = function (error) {
      console.log('Error: ', error);
    };
  }

  form_el.addEventListener("submit", function(e) {
    form_el.action += operation.value;
  });

</script>