This will send the command 0x4 followed by the command 0x20.
./onkyo-ri-send-command 0 26 0x4 0x20
The first argument is the number of a gpiochip device. The second argument is the number of a GPIO line. Both can be found in the output of gpioinfo. On a Raspberry Pi 3B chip 0 / line 26 is GPIO26.
nix-shell
make
0x4 Toggle power
0x20 Line 1
0xd9 Power on
0xda Power off
0xe0 Line 3
0x170 Line 2
0x203 Volume up
0x303 Volume down