How to force Forticlient v5 MAC address ?

Hello everyone,

I'm trying to configure Forticlient v5.6.0.1075 with DHCP over IPsec. On the VPN concentrator (a Fortigate, v5.4.4), I activated DHCP relay to route requests to a centralized DHCP server.

I noticed that the Forticlient uses the MAC address of the first interface used by the Forticlient to connect to the VPN (since last reboot of the computer). I want to force it to use the MAC address of its wired ethernet NIC, regardless if it's using another interface (like the Wi-Fi), because I want to manage just one entry per Forticlient on my DHCP server.

Doest anyone knows how to do that ?