filling fields from pop up window

I open a pop up window using javascript, I want to fill the form in the parent window on click of that pop up window using javascript.

Here is pop up window code, now on click on pop up window I want to fill form in parent window, how can I do this?

<script type="text/javascript">
function openPopup(url)
{
	window.open(url, "popup_id", "scrollbars,resizable,width=400,height=400");
	return false;
}
</script>
<a href="aircodes.php" onclick="return openPopup(this.href);">Airport Code</a>

Code (markup):

 

Hi

I am not having so much knowledge of Javascript.if u find the answer also tell me

Thanks