Perhaps you could also try this:
document.getElementByName("customer[url]").value="";
Not sure, as I'm not a JS guru. If that doesn't work, I'd assign that field an id ('<input ... id="myTest" ... >') and then use:
document.getElementById("myTest").value="";
As leatherback said, however, please note this isn't a Javascript board; don't expect Javascript guru's to be present (though there may be some around...)