Since one user was not able to get automatic activation working due to the inability of OSM Scout Server to create systemd socket, I am pasting contents of the required files here.
The files have to be put into /home/nemo/.config/systemd/user (see end of https://rinigus.github.io/osmscout-server/en/ for details).
[Unit]
Description=OSM Scout Server
[Service]
ExecStart=/usr/bin/harbour-osmscout-server --console --systemd --quiet
[Unit]
Description=OSM Scout Server Socket
[Socket]
ListenStream=127.0.0.1:8553
[Install]
WantedBy=user-session.target