Everything about starting a Xfce session is defined in /etc/xdg/xfce4/xinitrc.
In case that this property is not already defined is best to use -n (create) option and then we must also provice the -t (type) for that new property (which will be bool).
/usr/bin/xfconf-query -c xfce4-session -n -t bool -p /startup/screensaver/enabled -s false
Now, if you think that this article was interesting don't forget to rate it. It shows me that you care and thus I will continue write about these things.