Thank you for your post.
I don't know of a way to directly POST data to a URL. The only suggestion that comes to mind is use a Webviewer to display the information (be sure to check the option to allow interaction) and press the Submit button. I know this isn't the ideal solution, but thought it should be mentioned.
I have the feeling I'm overlooking something... Anybody have a better workaround and/or solution?
On a Mac, you could call AppleScript to run a shell script using cURL. I suppose something similar should be possible on Windows, but I don't know much about that.
This, BTW, deserves to be a feature request - especially in the context of importing external XML data. The inability to specify a POST request as the import source file closes an entire world of web services to Filemaker users.