no mobo made uses or is there a standard for mobo power switch nor power led status, all  are custom to the OEM , but if you had Asus.com or msi,gigabyte mobo , the manual  covers this 100%, not dell they do not sell DIY mobo, ever.

you started the post not even telling what's up doc.> what are you doing? and starting and goaL.

I guess a full Dell PC not missing major parts? but which one?

  3020 mobo (MT,DT,SFF,???? ) mother board  no PC size stated,  at all,  below I will answer for MT only.

the smart switch does not know what PC it is installed into ,for sure.

the answer is here.(power pins documented)

pin 3 to pin 6 is the power on pins.  on this dell.

but you just unpugged the  dell cable and wonder why dell reports it missing well that is 100% normal

that is dells way of telling you gee the cable fell off put it back now.

what is the solution is learn to solder, and using heatshrink tubing, do the job right, clean and pure.

no twisted wires with black electricians tape mess.

buy a spare 3020 power cable switch , step 1. (no case told so no answers possible, case size)

then cut the 2 wires that are P3/6 and then connect those 2 from mobo side pair wires to your new

smart switch,  now when you power on the PC the poor PC does not see dead power LED pairs.  bingo fixed.

use a spare cable and not wreck the original cable (for later use or sale of PC,.   )

good luck with that funny card.

The pinout for the 3020 power button is not documented. If specific pins are not attached there will be a front panel error. 

RMMW2 Optiplex 390, 3010 Power Button.

3020 is probably similar.

Dell Optiplex 3020 Power Switch LED Cable YPX0C 0YPX0C

https://www.dell.com/learn/us/en/04/videos~en/documents~esupport-optiplex-390-mt-power-switch-cable-en.aspx

The Dell 3020/7020/9020 MT models use Proprietary Connectors on its motherboards and supporting parts (Power Supply, Front I/O Panel, Power Switch/LED).

The 5-pin (2x3) header on the motherboard uses  Dell P/N: 30WGC or similar Power Switch.  I suspect its the same for the 3010 3020 etc., Power LED, and Diagnostics. This cable uses a momentary switch and a Multi-colored (white, orange) LED.

Power Switch (momentary) function: Black and Yellow wires. Power bi-color LED (White / Orange): Red and Green Wires

This Dell Optiplex 0x20 MiniTower series uses White - Normal and Orange - Sleep or Diagnostic (blinking) bi Color LED.  YMMV.  PIN 1 AND 2 shorted permanently stops front panel error.  YMMV

POWER BUTTON

OPTIPLEX 7010 9010 MT MINITOWER POWER BUTTON CABLE 030WGC

I can tell you what it is.

The wire colors have NOTHING TO DO with voltages.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.  i believe pins 1 and 2 must be permanently shorted to bypass the front panel error.  YMMV

In the usb  2 header on the 3020 sff you need to short pin 8 to pin 10 in the new connector and the error disappears