Install

$ doas pkg_add xfce

In /etc/X11/xinit/xinitrc, comment out these lines:

# start some nice programs
xclock -geometry 50x50-1+1 &
xconsole -iconic &
xterm -geometry 80x24 &
fvwm || xterm

and add this line underneath:

startxfce4                                   

Then $ doas rcctl restart xenodm

NOTE: For some systems, editing /etc/X11/xinit/xinitrc does not seem to work, but this command does work:

echo 'startxfce4' > ~/.xsession

Then restart xenodm:

$ doas rcctl restart xenodm