Can the ASA can authenticate portal access on the firewall (non-firepower)?
The ASA would have to intercept a https/http connection request to the public IP of the portal and shunt the request to an authentication service before allowing the traffic.
In other words, publish an internal and/or DMZ based website with one-to-one NAT to a public IP on the outside interface. The outside/dmz rules control what is allowed. Then have access authenticated by an artifact other than the webserver itself such as a radius server, Okta, or DUO.
I did not think this was supported.
