How do I auto-populate a text field from URL Parameter using Javascript

You can use location.search to access the url query and inject it into your text value

If your url is /page.htm?x=y, you can use the following code to set the value of a text field. This post tells you how to grab values from the query string

// See the link above for getUrlParams
var params = getUrlParams();
document.getElementById('myTextFieldId').value = params.x;

Notice that the example you've linked to suffers from a http://en.wikipedia.org/wiki/Cross-site_scripting vulnerability. To avoid that, you must escape the HTML before you output it on the screen

You should be able to do something similar using windows.location in js

http://css-tricks.com/snippets/javascript/get-url-and-url-parts-in-javascript/

You can easily use the answer of this question to get parameters from the URL using JavaScript. Then set the value of your textboxes like this:

this.form.elements["element_name"].value = 'Some Value';

If you want a quick solution, you might be interested in using this code snippet (inspiration taken from here, but modified by myself):

function getURLParameter(e) {
  return decodeURI((new RegExp(e + "=(.+?)(&|$)").exec(location.search) || [, ""])[1]);
}

if (getURLParameter("inputKey") === "") {
  console.log("No URL param value found.");
} else {
  document.getElementById("inputBox").value = getURLParameter("inputkey");
}

Let's say that your url is example.com?inputKey=hello world. If so, using the above code will fill in the input with the ID of "inputBox" with the phrase "Hello world". If the URL was just example.com, it wouldn't prefill the box, and instead just act as though there was nothing (show the placeholder etc).