Open up a terminal and exebute
Then you can edit the sudoers file where you can specify who can execute which command as root with and without password. At the end of the file you paste either this line
for every user to be able to execute that command as root without password, or
for only your user to be able to do this (of course you have to adjust it for your user). Then you save the file (should be Ctrl+O) and exit the program (Ctrl+X).
At last, you have to edit your command to read
sudo pm-suspend-hybrid instead of
NOTE: There should also be the possibility to do this with dbus (doesn't require sudoers editing) but this solution has the advantage that it works with every UI.