Grab Parameters and set form field values

params_field_values.html

<script>
//Function that stores parameters in array, returns parameter value
  var getUrlParameter = function getUrlParameter(sParam) {
    var sPageURL = decodeURIComponent(window.location.search.substring(1)),
        sURLVariables = sPageURL.split('&'),
        sParameterName,
        i;
    for (i = 0; i < sURLVariables.length; i++) {
        sParameterName = sURLVariables[i].split('=');
        if (sParameterName[0] === sParam) {
            return sParameterName[1] === undefined ? true : sParameterName[1];
        }
    }
};
  
   var firstValue = getUrlParameter('first'); //Replace 'first' with name of your parameter
   var secondValue = getUrlParameter('second'); //Replace 'second' with name of your parameter
  
  $("#coolform input[name=firstname]").val(firstValue); //Replace #coolform with ID of your form, and firstname with name of your field
  $("#coolform input[name=lastname]").val(secondValue); //Replace #coolform with ID of your form, and lastname with name of your field
</script>