First you need to create a mapping that redirects requests to a URL:
Then you need to create a rule like this:
Source: Any IP address : Any port
Destination:
not X.X.X.X : port 80
Protocol: IPv4 TCP
Mapping: Test
Where x.x.x.x is the address of your portal. After this, whoever attempts to open a web-site and is not processed by a preceding rule, will be handled by this rule and redirected to the portal.
It's important to exclude the portal's address in this rule as otherwise in may result in a endless redirecting loop.