Here’s how to enable Wake-On-LAN on a Linux system with systemd:

1. Find out interface and MAC address:

> ip a

2. Check if Wake-On-LAN is already enabled (d is disabled, g required):

> sudo ethtool <interface>
...
Wake-on: d
...

3. Create a new file /etc/systemd/network/50-wired.link for systemd-networkd:

[Match]
MACAddress=<MAC address>

[Link]
NamePolicy=kernel database onboard slot path
MACAddressPolicy=persistent
WakeOnLan=magic

Sources: 1, 2