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 193316
If there is requirement to allow only the Anydesk traffic or if Anydesk traffic effected by the SSL inspection, create the dedicated IPv4 policy with allowing only Anydesk traffic using ISDB feature under IPv4 policy configuration.

This article describes how to allow it.

The Internet Service Database is a comprehensive public IP address database that combines IP address range, IP owner, service port number, and IP security credibility.
The data comes from the FortiGuard service system.
Information is regularly added to this database, for example, geographic location, IP reputation, popularity & DNS, and so on.
All these information helps users define Internet security more effectively.
Use the contents of the database as criteria for inclusion or exclusion in a policy.

Here is example of allowing Anydesk traffic using ISDB under policy.

To apply a predefined Internet Service entry to a policy using the GUI.

1) Go to Policy & Objects and select 'Create a New policy'.
2) In the  Destination field, select '+' icon.
3) In the Select Entries pane, select 'Internet Service'.
4) Locate and select 'Anydesk-Anydesk'.

5) Configure the other fields and then select 'OK'.

Keep this Anydesk policy at the top of other generic policies to take effect.