You'd define it as a movement key as you want the keypress to be held by HKN while the hotkey is pressed. You'll also need to define Lshift to be a modifier key before it can be used as such in the script.

These are the 2 relevant documentation pages:

http://www.hotkeynet.com/ref/usekeyasmodifier.html

http://www.hotkeynet.com/p/movement.html

Note that you don't actually need to be sending Shift+Space to the clients. You can have that be the hotkey that you press and have HKN just send Space.