This link should work without problems - iOS devices should be able to understand it. Don't try to use it from your desktop. However, it might be impossible to write it into Safari's address bar.
If that's is the case, create a simple redirect HTML:
<html>
<head>
<title>Simple redirect</title>
<meta http-equiv="refresh" content="0;URL='itms-services://?action=download-manifest&url=PLIST_URL'">
</head>
<body></body>
</html>
Also make sure the URL parameter is properly encoded.