Marlon, the external switches/wireless controllers are typically used to write a CoS value so a external policy-based router can look at the http packets from a specific initial role. They get redirected to a portal, but not a specific portal page. The NAC will then decide based on login criteria and assessment results which page to display.
So I am uncertain what specifics you are trying to accomplish.
Here is an overview of the Redirect process for the NAC.
https://gtacknowledge.extremenetworks.com/articles/How_To/How-To-Determine-What-Type-of-Redirection-...
