I have a link like this ... test_popup.asp?s_Postcode=1

My question is how do I grab this s_Postcode variable. The grabing page look like this at this time.

Using alert, I can see the whole URL but how do I put that s_Postcode into my textbox?

<html>
<script language="JavaScript">
function get_parameter() {
var strReturn = "";
var strHref = window.location.href;
if ( strHref.indexOf("&") > -1 ){
var strQueryString = strHref.substr(strHref.indexOf("&")).toLowerCase();
var aQueryString = strQueryString.split("&");
for ( var iParam = 0; iParam < aQueryString.length; iParam++ ){
if (
aQueryString[iParam].indexOf(strParamName + "=") > -1 ){
var aParam = aQueryString[iParam].split("=");
strReturn = aParam[1];
break;
}
}
}
return strReturn;
}
</script>
<body onload="get_parameter()">
<form name=FrmEditContact>
<table>
<tr>
<th>Postcode</th>
<td><input maxlength="12" size="12" name="s_Postcode" value=""></td>
</tr>
</table>
</form>
</body>
</html>