Just use XFCE’s power managment GUI configurator.

But, If you are felling adventurous try this,. Created a file (shell/script) at;
/etc/X11/Xsession.d/startup-local

#!/bin/sh
xset -dpms &
xset s noblank &
xset s off &

Made it executable using:

chmod 755 /etc/X11/Xsession.d/startup-local