How to specify command to always run as root?
You could use a Run AppleScript action that runs do shell script "purge" with administrator privileges. It requires entering a password though.

If you don't want to enter a password, add %localaccounts ALL=NOPASSWD: /usr/sbin/purge to sudoers. (Run sudo visudo, press i, type or paste the text, press escape, and type :wq.) Then use a Run Shell Script action that runs sudo purge.