StartPage.html

<!DOCTYPE html>
<html>
  <head>
<script>
// Prevent forms from submitting.
      function preventFormSubmit() {
        var forms = document.querySelectorAll('form');
        for (var i = 0; i < forms.length; i++) {
          forms[i].addEventListener('submit', function(event) {
            event.preventDefault();
          });
        }
      }
      
      window.addEventListener('load', preventFormSubmit);
      
      function handleFormSubmit(formObject) {
        google.script.run.withSuccessHandler(updateComputation).processForm(formObject);
      }
      
      function updateComputation(computation) {
        var div = document.getElementById('output');
        div.innerHTML = computation;
      }

    </script>
</head>
<body>
<form id="myForm" onsubmit="handleFormSubmit(this)">
Starting array value 
  <input type="text" name="startVal"><br/>
  <input type="submit" value="Generate Computation"/>
</form>
<div id="output">
  computation here
</div>
</body>
</html>