If you have a need to always redirect a WLAN to a static webpage, such as for Wi-Fi setup instructions where you have no intention of the user authenticating to the portal, you can either use the built-in Mist portal and customize the layout to your liking. Or if you have an external webpage already built, you can leverage forward to external portal.
If you use an external webpage, be sure to add any external dependencies there may be to load the page into the allowed hostnames box. So that way the page will load properly. You can easily discover external dependencies by visiting the actual url with browser developer tools opened and looking at the network tab.
External portal configuration example, with allowed hostname.
