Hi Rob,
This could be achieved with the use of Software Files and Actions tab that is present on the task Window. Once you select that you would have many choices to select from. On that window, you could select "EXECUTE COMMAND" option and execute the shutdown command. As for the startup, when the client machine is turned off it would show as disconnected and hence it would not be possible to run a task. There is a possibility of using Wake On Lan for these purposes.
Hope this provides some insight and answers your questions !