What does "P" do?
You can't setup/start a countdown using a hotkey. You can configure/start a countdown from the Game tab (Countdown button on the left).
You could also create an event that fires when the countdown starts. So you could then configure your countdown, press the spacebar to start the countdown (or if you really wanted it to be the "P" key, change the hotkeys so that P starts/stops the clock), your event fires (and a sound plays), and the tournament starts at the end of the countdown...