Xscreensaver on startup, i3wm

if I understand it you want to put xscreensaver after starting i3wm ? if you enter into the i3wm config file as the last line exec xscreensaver

Yes, usually just

xscreensaver &

will do what you want. But it depends on what else you have in your .xinitrc (or .xsession) file. For both of those, you need to leave some program running in the foreground to prevent the script from completing (and stopping X).

If you have a workable script, when adding your command before that final program should work.

Further reading:

• How do xsession, xinitrc and .i3/config play together?
• e X Window User HOWTO: 3. Running X