JavaScript Validation

System

I have a APIForm that has two linked drop down list boxes that works. But I would also like to insert Javascript into this file (or other file) where I can validate form entries acrosss multiple fields. So far, I havent been able to get a customized JS alert box to appear. Is this possible via Teamsite ?.

Code sample:
function init()
// Validate some entries on the form
var filetype = IWDatacapture.getItem("/File Type");
var filesize = IWDatacapture.getItem("/File Size");

if ((filetype.getValue() == "ppt") && (filesize.getValue() == ""))
{
alert("A file size must be specified for PPT files");
return false;
}
}