Pinout of the I/O panel is not forthcoming, not posted or given out by Dell any model.

The ATX paperclip grounding of the green wire to black wire with a normally open push button would work.

First, the switch makes momentary contact.

This means that a toggle-type switch will generally not work as a replacement power switch unless it’s also momentary.

Second, the switch is normally open, also known as push-to-make.

When you select a momentary switch, be aware some are normally closed, or push-to-break.
Third, the current carried by the ATX power switch is very low, so you may select almost

any switch you like without regard to the current rating.

This also means you can extend the wires to your power switch using inexpensive hookup wire.
Fourth, the voltage carried is no more than 5V, so you don’t need to pay attention to the voltage rating.
Fifth, the minimum switch you need is SPST (single-pole, single-throw), but any of the other

switch types can be connected to act as a SPST switch, simply by selecting the correct two leads.

Four wires have special functions:

PS_ON# or Power on is a signal from the motherboard to the power supply. When the line is connected to ground (by the motherboard), the power supply turns on.

Thank you for such detailed information.  Forgive me if I make a novice mistake in interpreting it.

I bridged pins 15 and 16 on the connector from the top so that it could remain on the motherboard.  It did work somewhat, because the power supply, fans,  and CD drive were powered, but the system itself did not boot.  I still assume that the mobo needs the switch signal via the I/O cable to actually kick off the boot.

Remember I don't really need to install a switch, but would installing one change my results in any way?  It feels like there's still something missing here.

It has to be a momentary button that is normally OPEN.  

SPST MINI MOMENTARY

This is a year old post about turn on an ATX supply, so I'm not sure the author is still monitoring this.  Seems like good info, but this doesn't work, as stated by a previous posts. This momentary connection of the green to black only turns on the PSU fans for me.  It doesn't boot the system.  It appears something else is missing. Interesting enough, the author says to use a momentary switch between the green and black (ground) wires, but his video and illustration shows a paperclip inserted for a constant connection, not a momentary connection.  So the instructions don't match the demonstration, which is a clue that something has been overlooked in the directions.

Yeah, unfortunately this trick did not work for me.  He did say I think that the paperclip would have to be replaced with a momentary connection, but even with the specific switch he suggested I had no luck.  I tried some other methods but I eventually messed up the power supply, so I ended up abandoning the project since it was just an old PC I was only using as a media server. The infuriating part is that the PC was fine, the power switch was just over-complicated..

The power switch is probably mounted on a circuit board, so you may need to replace that to get this to work. And there's always Dell Spare Parts.