Fortigate user authentication only for one destination

Hello to everyone on this forum, I`am new with fortigate, have some prior experience with different NFGW vendors.

I have following problem:

I would like to have two firewall policy for LAN.

First would be:

If someone from LAN want to connect to webpage http://www.example.com user have to authenticate through captive portal

Second rule:

Permit any other destination from LAN without authentication.

So far I only managed to:

-Enable captive portal on interface

-Authenticate every user, than per user create policy.

But the main point is that I want to show captive portal only to users, that want to go to destination http://www.example.com, for every other destination there is no need to authenticate.