This is a guess. Dell may be using old 5-pin power switch pinout they have used before.

If you momentarily  short pin 1-4 and system starts, it is verified.

The pin number labels don't match what you have in the screen, but there is the same number of pins. They are numbers bottom to top left to right. I.e....

246

123

Anyone know for sure? And which ones are LED power as well?

Power switch and Pwr LED pinout (old Dell 5 pin).  

XPS 8940 power switch 5 pin.

@Djstepo  - Can I ask why you want to add a 3rd party power switch?

disclaimer: above pinout to be verified.

verification of pwr sw should be simple. just short the “power switch pins” momentarily using flat blade screwdriver when power to motherboard is on.

The device is going into a pinball cabinet and need to route a long wire up to the front of the cabinet so it can be controlled easily.

@Djstepo  Why don't you just make life easy and avoid all the mods?

Plug PC and monitor into a surge protector or power strip that has its own on/off switch.

Now boot PC and open BIOS setup by tapping F2 at the Dell splash screen. Set the AC Recovery option to "Reboot" after a power failure. Don't change anything else in setup, but save the AC Recovery change and exit setup.  After you shut PC down normally in Win 10, turn the surge/strip off at its own switch.

The next time you turn the surge/strip on, PC thinks it's recovering from a power failure and boots itself without having to press its power button, and the surge/strip can also turn the monitor etc on at same time.

I've done this for years with my PC, monitor, speakers, printer, etc. One switch turns everything on all at once and PC boots automatically.  Just make sure the surge/strip can handle the load for all the devices connected to it since they're all powering on at the same time.

And you can call me lazy!

Thanks @speedstep this was perfect. 

@RoHe this is for a custom pinball cabinet project and I want to integrate a nice power button up front on the cabinet. So your solution would be functional but not aesthetically pleasing :).

@Djstepo  - I'm sure you could find a way to put a small in-line switch ahead of a surge or power strip and mount that on the cabinet, the same way you're planning to mount a switch wired directly to the PC...

And I also wonder if BIOS is going to throw an error message if you bypass the PC's own power button with a second switch wired in...

@Djstepo  So now I see you are getting a BIOS warning after bypassing the PC's power switch, as I suspected you would.. 

May be time to rethink putting a power switch ahead of a surge or strip and setting PC's AC Recovery to reboot.

I just wanted to verify that @speedstep has the correct solution for sure. I recently got an XPS 8940 to use for virtual pinball, and just like @Djstepo , I wanted to have an arcade button on the machine turn the PC on and off. I used the amazon link from @speedstep , cut off the button assembly end, soldered red and yellow to a single wire, and then ran that wire to one end of an arcade button, and the black wire to the other end of the arcade button. All worked perfectly, and no bios warnings since red and yellow are shorted. Thank you all, as this was exactly what I needed!

@Djstepo 

@stazna01 

@RoHe 

Harbin Repairs has a github site with schematics so you can make your own adapters.  They also sell them.

harbinrepairs power adapter

https://www.youtube.com/watch?v=3fb6-B5Ux_s

optiplex 390 3010 3020 front-panel  adapter kit

Get it on eBay:

https://www.ebay.com/itm/264638815586

https://www.ebay.com/itm/264638806788