No harm will come if you guess, or if you guess wrong.
The power sw does not have polarity(+/-).
In your motherboard manual, look for the front panel header description.
There should be two pins for the power sw. It can be connected either way. It will be attached to a momentary pushbutton that you use to power on the pc. You can leave the others disconnected if you need to.
One will probably be a led that lights up when power is on, or one that is for hard drive activity.
+ and - is relevant to led switches, If you get them backward, they just don't light up. reverse them.
The last possibility is a reset switch, It is a momentary switch like the pwr button.
Pre built pc's often have a front panel header with all the switches and led's in one block. Most likely that block will not fit your motherboard arrangement of the front panel header. There is no standard. If needed, you can carefully separate the pins by cutting the plastic between them.
If necessary, just run without a power on light, hdd activity light, and reset switch.