Many of the editing commands in Virtuoso (e.g. copy, move, stretch etc) allow both pre- and post-selection. The idea is that if you have anything selected before you start the command, it will operate on what is selected. If nothing was selected, it will prompt you to select the items to operate on.
You can also partially select objects in advance (hit the F4 key to toggle between partial and full selection modes).
So there's nothing special you need to do in the command to use either pre- or post-selection; it's purely dependent upon whether anything was selected when you start the command.