This is the pin:
That picture answers your question - it's the 5th pin from the left. Also confirmed in tawys thread: https://www.pcenginefans.com/forum/index.php?topic=23590.msg524226#msg524226
Wire that to GND and you are good to go.
From the photo I was in doubt, because in the image there is a red circle, identifying the point, but this does not go to the 5º pin.
I should solder on the blue dot (it's connected to the 5º pin) or I should solder the red dot (it's connected to the 6º pin).
I also looked at the other link you shared but I didn't find that information. At least I didn't find it.
Anyway, very grateful for the answer.