Yesterday I wrote an article about some advanced policy option in FortiOS 6.2.3. Here is another one. In System then Feature Visibility select the button for Policy Disclaimer


This enabled the following option inside the policy section

You can now choose on a per-policy basis, a customized message group to use. In 2019, I wrote an article about using custom messages in Fortigate.

Here we can see where the Policy object is pulling from.

Got to (1) System, (2) Replacement Message Groups, (3) Group
Hope this helps