How do I run a launchd command as root

"What folder is the .plist stored in?

launchd runs Daemons (/Library/LaunchDaemons or /System/Library/LaunchDaemons) as root, and will run them regardless of whether users are logged in or not. Launch Agents (/Library/LaunchAgents/ or ~/Library/LaunchAgents/) are run when a user is logged in as that user. You can not use setuid to change the user running the script on daemons.

Because you will want to add it in /Library/LaunchDaemons you will want to make sure you load it into launchd with administrator privileges (eg. sudo launchctl load -w /Library/LaunchDaemons/com.apple.samplelaunchdscript.plist)

Check out man launchd for more information."

"Property lists in LaunchAgents also work, but you have to load both agents and daemons with sudo:

sudo chown root /Library/LaunchAgents/test.plist
sudo launchctl load /Library/LaunchAgents/test.plist

If the plist doesn't have a disabled key, it is loaded on the next login or restart by default, and -w is not necessary.

Technical Note TN2083: Daemons and Agents:

A daemon is a program that runs in the background as part of the overall system (that is, it is not tied to a particular user). A daemon cannot display any GUI; more specifically, it is not allowed to connect to the window server.

[...]

An agent is a process that runs in the background on behalf of a particular user. Agents are useful because they can do things that daemons can't, like reliably access the user's home directory or connect to the window server."