I already have the code, I guess I just don't know how to properly execute it in the form itself

<script type="text/javascript">
<!--
function getNS_url_param( name ){ 
name = name.replace(/[\[]/,"\\\[").replace(/[\]]/,"\\\]");  
var regexS = "[\\?&]"+name+"=([^&#]*)";  
var regex = new RegExp( regexS );  
var results = regex.exec( window.location.href );  
if( results == null )    
return "";  
else    
return results[1];
}
function trackNSParams() {
var name = 'leadsource';
var value = getNS_url_param(name);
if (value != null && value.length > 0)   
document.cookie = name+"="+value+"; path=/";
}
function appendNSParams(url) {
var nameEQ = "leadsource=";
var ca = document.cookie.split(';');
for(var i=0;i < ca.length;i++) {
var c = ca[i];
while (c.charAt(0)==' ') c = c.substring(1,c.length);
if (c.indexOf(nameEQ) == 0 ) 
      url = url + '&' + nameEQ + c.substring(nameEQ.length,c.length);
}
return url;
}
// -->
trackNSParams();
</script>