Post-authentication redirection using continue form item
This information is also available in the FortiOS 8.0 Administration Guide:
FortiOS now supports the continue form item from external captive portals, enabling post-authentication redirection to a specified URL after successful user authentication.
When a client generates a POST request for authentication that includes a continue form item (CONTINUE_URL = "<web site>"), FortiOS ignores the security-redirect-url option configured in the captive portal and redirects the user to the URL specified in the continue form item instead.