You don't need the APU to start the harrier.
Also, EFC should be in pos 2 (https://forums.eagle.ru/showthread.php?t=208581&highlight=pos%2A2&page=3)
The only things needed to be pressed for startup (in dcs):
Battery
Fuel lever
DECS Switch
L and R fuel pumps
Engine start switch
at around 9% rpm: throttle to idle.