Ok I'll try to describe it to you. Couldn't find anything about this model but everyone says it's similar to other HP server PSUs. Check if you have pin numbers written on the board (i can see that the numbers re slightly different in this model) Check for pins 30 and 31.
Pin 30 is a voltage sensor and should be connected to +12V
Pin 31 is a power switch just like in typical PSU and should be connected to the same pin on the other side of the board (pin 34).
In case you don't have it written I found another way.
As I wrote before you need to find 2 pins that turn it on.
One is called PSKILL (this should be the short pin) and it should connect to the ground (usually it's the same pin on the other side of the board but check it first)
The other one is PSON and here I'm not completely sure as I've seen people connect it to +12V but i also saw a schematics showing it turns PSU on when kept between 0 and 1.2V. This is probably the pin on the right of PSKILL, connect it to e.g +5V and use a variable resistor starting from 0 to find the correct voltage.
I hope this helps