fetch-url-params-and-set-hidden-field-values.js

var setHiddenInputValues = function(formId, urlParams) {
  var inputs = document.getElementById(formId).elements;
  inputsArr = Array.apply(null, inputs);

  hiddenInputs = inputsArr.filter(function(i) {
    return i.type === "hidden";
  });

  // set value of inputs to the url params passed in
  hiddenInputs.forEach(function(i) {
    key = i.name;
    val = urlParams[key];
    if (val) {
      i.value = val;
    }
  });
};

var getParams = function(url) {
  var params = {};
  var parser = document.createElement("a");
  parser.href = url;
  var query = parser.search.substring(1);
  var vars = query.split("&");
  for (var i = 0; i < vars.length; i++) {
    var pair = vars[i].split("=");
    params[pair[0]] = decodeURIComponent(pair[1]);
  }
  return params;
};

//Using the above functions to parse the url params and set the hidden form fields to their values
$(function() {
  var url = window.location.href;
  var urlParams = getParams(url);
  var formId = "wf-form-Contact-Form";
  setHiddenInputValues(formId, urlParams);
});