It seems I need this in .xinitrc:
.xinitrc
if [ -z "$DBUS_SESSION_BUS_ADDRESS" ] && [ -x /usr/bin/dbus-run-session ]; then exec dbus-run-session -- $0 "$@" fi
https://www.linuxquestions.org/questions/slackware-14/current-notify-send-with-fvwm-or-any-wm-4175706394/