I'm not sure if this is intended or not but the webstart link builder doesn't work to launch a file if you have the file on a dropbox folder or Google drive.
It's not a major issue as I've found github to be a viable alternative.
If you can get a direct http link to the IOK file (rather than a link to the page with a download button), it will work. Otherwise you should stick with the alternative.
Only the Public Folder of your Dropbox has an option to generate a public-facing URL you should be able to use. You can certainly use this method to send IOK files in your Public Folder to anyone via a the Dropbox-generated link in an e-mail...no website required!