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 192133
This article explains how to configure captive portal for LDAP user.
First thing, configure the LDAP Server:

Go to User & Device -> LDAP Server
Select 'create new' and configure as following:

The second step is to configure the user group to use:
Go to User & Device -> User Groups
Select 'create new' and configure as following:

Choose a specific group if required.

Then go to the interface to enable captive portal (wire or wireless)

In 'Admission Control', 'Security Mode' choose 'Captive Portal'
Change 'User Access' to 'Restricted to a group'
In 'User Groups', choose the LDAP user

Create the policy to permit access to this group to the required resources and test the configuration.
The captive portal will be displayed.

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