I used to use XFCE (it was with OpenBSD 5.4).
By the time, to shutdown the computer from XFCE I added one of these two lines to /etc/sudoers:
Code:
username ALL=NOPASSWD: /usr/local/lib/xfce4/session/xfsm-shutdown-helper
or
Code:
%users ALL=NOPASSWD: /usr/local/lib/xfce4/session/xfsm-shutdown-helper
On the 1st line, username is to be replaced by your user name.
For the 2nd one, you need to belong to the user group.
If xfsm-shutdown-helper still exists, maybe the same thing can be achieved with doas.