It seems to me you are trying to set up Captive Portal with External Server, which currently we are supporting HotspotSystem and PurpleWiFi.
In the coming firmware 7.1.1 (which is in BETA now), we will be supporting CoovaChilli mode that allows the customization of the splash page, including customization of the attributes. If the external server supports CoovaChilli mode, then likely it should be working.