FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
Article Id 192801
This article explains how to redirect all computers within the network to a specific URL upon starting of Internet Browers.

For example, a guest walks into meeting room with a laptop, enables the "Unprotected WiFi" which is the gateway controlled by the FortiGate. When the guest tries to enter "", they will be redirected to "" for the first session.

This redirection can be obtained by enabling a disclaimer in the firewall policy so that the firewall will interrupt traffic and redirect it to a specific URL.

Create a firewall policy with id 1 from internal to wan and then use the following command:
FGT# config firewall policy
FGT# edit 1
FGT# set disclaimer enable
FGT# set redirect-url ""
FGT# end