Hi Helle,
You'll need to write code for this, and pretty much any language or environment you or someone in your team is familiar with will do. The gist of it really is just to retrieve the HTML for the page by making an HTTP request to Orbeon Forms, just like your browser does when you load the form.
But, if you haven't done so already, you might want to check with the owners of that web service that they will accept that HTML, as it is somewhat unusual for a web service to consume HTML, especially when it is produced by a third-party application.
Alex
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