New External Switch

[MarkHaysHarris777]

So - red on Pin 5 of the EXP headers, and black on Pin 6?  For power on?

And I'm assuming the pins, when looking at the board with EULER and EXT at the bottom, are arranged thus :

2-4-6-8-10
1-3-5-7-9

hi , in answer to your first question, yes.
... pin(5) is the power|stby switch signal and pin(6) is ground.  It does not matter what color the wires are that you use to connect these pins to your momentary 'ON' switch. The point is that the switch 'momentarily' shorts those to electrical points together [ pin(5) and pin(6) ] activating the power OFF or Standby.
In answer to your second question, you don't have to assume anything.  The pins are labeled right on the board. Well, only the corner pins are actually labeled. And yes, you understand the pins correctly, just as you have depicted them above, if you are looking down at the EXP header from above with the pins at the bottom of the card nearest you.
hope this helps you