SDDM is a modern display manager for X11 and Wayland aiming to be
fast, simple and beautiful. It uses modern technologies like QtQuick,
which in turn gives the designer the ability to create smooth,
animated user interfaces.

You will need to edit /etc/rc.d/rc.4 to get this work.
Put the section that refers to sddm before the sections for xdm, kdm
etc:

# Look for SDDM as well:
if [ -x /usr/bin/sddm ]; then
  exec /usr/bin/sddm
fi

Note: The home for SDDM is set by default in /etc/passwd as
/var/empty. This ought to be changed to /var/lib/sddm. To do this,
run:

usermod -d /var/lib/sddm sddm

when SDDM is *not* running. Do this before installing or upgrading the
package.

Note that this is fixed in -current.