Assigning Unique Local Addresses to the loopback interface

I have Ubuntu 18.04.2, upgraded from 16:04. Previously I had an IPv6 ULA (Unique Local Address) assigned to the loopback interface in /etc/network/interfaces. I tried to make the same assignment in netplan. Declaring “lo” as an Ethernet interface is accepted by the parser, but it has no effect. How can I assign a ULA to the loopback interface at boot time using netplan? Note: network manager is not mentioned as renderer, so I assume that systemd is operating here.