I need some help.

I'm currently making a tool to check username availablity. (RuneScape)
It's used to create a account. I've created it in a web page. I've created a Form where you can enter a username to try it. I'd like to have JavaScript to make it that way, that if you go anywhere else with your mouse, (outside the input box.) that it will already say the answer. I know this is possible because Jagex Creation form has something too. The JavaScript is NOT added to my script yet because i do not know how to write it. So, it will actualy check if the username is available and you can go to the next step what is the new password.

Code:

<form action="nextstep.php" method="post">
<div class="formstyle">
<input id="username" name="username" autocomplete="off" maxlength="12" value="">
</div>
<div class="button">
<input type="submit" value="Check Username">
</button>
</div>
</form>

Okay, Here is what i do know:
-This RuneScape page provides username checking: https://create.runescape.com/checkusername.ajax?username=str
str is username. And if the first 2 digits in the document are 17 it is available and 18 is unavailable.
-You can create a script with JavaScript to check the username's availablity before clicking the submit button.

Well, Can anyone create a script for me or help me?

 

I do not do JS but I can tell you 47 means that you are connecting too fast and it refused you.