Newgrounds uses a content delivery service so there is no static url.
It will most likely change each time you upload a new version, and it will take several days before the old files are removed.
You can obtain what the current url is by opening the html source, and searching for the iframe tag.
As is you have no way to know what the url will be before you upload.
A possible solution would be a combination of Magistross s code with a trigger to fetch the correct url from an external source that you have access to.