odroid-h3:application_note:external_power_switch [ODROID Wiki]

You can turn on or off the ODROID-H3 with the external power switch instead of the onboard switch (PWR_ON1).
The power pin (PWR_BTN) is exposed to the pin #17 on the pin header (EXT_HEAD1) and this pin needs to be shorted to GND momentary.
Please note you can wire up the momentary on-off switch only, otherwise, the ODROID-H3 will keep turning on and off.

There is a small power switch on the ODROID-H3 board. But it is not easy to access the switch when you assemble(place?) the board in to a case.
This beautifully illuminated power button is a good solution what you need!

This button can be mounted on all of the official acrylic cases Type-1 to Type-7.
Note that the blue LED forward voltage must not exceed 3.5Volt.

Red wire: LED positive (Anode) : pin-4 or pin-14 on 24 pin header of ODROID-H3 (Warning: pin-14 will always make the LED turns on even in the power-off state)
Black wire: LED negative (Cathode) : GND(pin-1) on 24 pin header of ODROID-H3
Green wire : Switch-A : #PWR_BTN(pin-17) in 24 pin header of ODROID-H3
Blue wire : Switch-B: GND(pin-19) on 24 pin header of ODROID-H3

This blue illuminated power button has a built-in LED which turns on when the system power is on.
Since the pinout #4 supplies “+3.3V” when the ODROID-H3 is turned on, you can wire up the VCC on the switch to this pin.
If you need to use “+3.3V” for another device, you also can use UART0_RTS# alternatively.