"systemd-networkd supports MAC address spoofing via link files (see systemd.link(5) for details)."

To set a static spoofed MAC address:

nano /etc/systemd/network/00-default.link
File does not exist, exist other? ls -l /etc/systemd/network

inside file:

MACAddress=original MAC

MACAddress=spoofed MAC
NamePolicy=kernel database onboard slot path

To randomize the MAC address on every boot, set MACAddressPolicy=random instead of MACAddress=spoofed MAC.

source: https://wiki.archlinux.org/index.php/MAC_address_spoofing#systemd-networkd

How to do it via GUI (possibly better/easier way):

right click Network Manager icon, Edit connections, click connection edit, "Cloned MAC address" field on the Ethernet tab has option of permanent, random MAC.