The problem is PBO doesn't have a power (standby) button instead it has a power switch.
Luckily the standby button switch is available (unconnected) on the PBO main board, you only need to add the physical swtich.
First you need to locate the switch (credit to wigout and snappy46):
Remove the PBO from its metal enclosure (you can turn the board over no need remove the fan mount, check snappy46 pic) and locate switch SW3 on the PBO board.
Then follow the steps below (credit to si1verfish)