Carl, you need to use Virtual Merchant - Merchant Provided Form.
You do not need to submit URLs anywhere. Demo URL is pre-coded in X-Payments integration with Virtual Merchant - Merchant Provided Form. You just need to enable "Demo" mode. See
http://www.awesomescreenshot.com/image/1052304/96ffdb4560e0c5e7063cec4c869b52db