There is no legitimate "keygen" for FortiGate VM licenses. Instead, Fortinet provides a official (starting with FortiOS 7.2.1) that replaces the old 15-day trials. This license is free, does not expire, and is intended for lab, study, and testing purposes. How to Get the Official Free License
Go to: support.fortinet.com → Register → Request Evaluation License fortigate vm license keygen free
Doing so would:
A more feature-complete evaluation license can often be obtained by contacting a Fortinet Account Manager Old Version Trials: Permanent Evaluation License There is no legitimate "keygen"