You can deploy FortiGate-VM (BYOL) outside the marketplace product listing using Azure PowerShell. This is an alternative method for if you want to deploy FortiGate-VM on instance types/sizes that are not found on the FortiGate marketplace launcher. Some instance types of your choice may not properly boot up or run due to lack of official FortiGate instance support.
You can also specify bootstrapping FortiGate CLI commands as art of a bootstrapping configuration file that is passed in PowerShell at the time of initial bootup.
That you have thorough knowledge of PowerShell and various Azure services and features to adopt this deployment method is expected.