sure you can , ever solder 2 wires and heat shrink tube them after? (during)??? a skill set.

lacking that the connectors are NOT the same nor is there a standard here, ever.

that gigabyte board, has manual that tells you the 2 pin that turn it on,  power on and ground

dell does not publish this, (a dark secret)

but my 3020 has 6 pins,  pin one is cut, and is the key pin1 (x) 2x3 pin matrix,with 1 pin cut off.(keyed)

so

123

456  (if I jumper pin 3 to 6 ground it turns on)  old mobo. (not saying 390 is this,but may)

so  wire the ground pin above (6) to your GB pin ground, power jack

then wire the 2nd pin called  pin 3 power on , to your GP power on pin.  and shrink it (means proper insulation)

done.

here is on other person asking same thing,  found it in 5sec. search this form.

https://www.dell.com/community/Optiplex-Desktops/Optiplex-5-pin-power-button-assembly-pinout/m-p/6242895/highlight/false#M30490

: Black and Yellow wires. if the old cable has colors some do not. ok?

so I told you : Black and Yellow wires.

and so did speed step.

: Black and Yellow wires.

if

http://pcdied.com/intel-magic/Dell/3020/3020.html#pwrbtn

here is the proof that the newer cable is all black, so colors don't matter,

what matters is pin names. colors are not on this cable, see dell PN there? 

I'm sure they saved 50cents going black only, (1/2 joking)

You will have to convert a cable.

https://www.amazon.com/Computer-Motherboard-Switch-Speaker-Antistatic/dp/B07NMPY6JZ/

OPTIPLEX 7010 9010 MT MINITOWER POWER BUTTON CABLE 030WGC

https://www.amazon.com/FidgetFidget-Optiplex-Button-Switch-030WGC/dp/B07M98CXY8/

I can tell you what it is.

The wire colors have NOTHING TO DO with voltages.

Another Way to look at it

Yellow Black is switch Red Yellow Shorted permanently.

POWER BUTTON WIRES

There are 2 wires for the on / off momentary button.

There are 3 wires going to a Tri color LED.

Careful disassembly of the cable makes this easy to figure out.

I am not however going to encourage this or provide instructions on how to do this. Others have indicated that the red and yellow wires are shorted together through the switch.

Not having ALL wires correct will have "ALERT! FRONT I/O CABLE FAILURE"  Orange Black Blue would be the Tri Color LED.  Educated guess says that the black wire is the common connection for the CENTER of the tri color LED.

https://www.youtube.com/watch?v=8Q0tvKB7tm4

I used your 5pin schematic and connected my switch as instructed, got a cmos jumper to short the other two pins, and also I got the LED to work by connecting the + wire to the 'orange' pin and the - wire into the cmos jumper. No error message, the switch works, and the LED works too