With this simple schematic you can make your PC turn ON when AC is connected, if this behaviour is not possible to achieve by tweaking your BIOS/UEFI. This is a custom circuit I designed for my particular laptop but it should work in most computers.
The blue area represents the circuit to build while the outside indicates the parts that are present in the mother board.
When power is applied to the circuit through R4, Q1 simulates the user pushing the power ON button. This pushing is stopped when the power ON LED is activated due to the computer being turned ON.
Note that the supply 3.3V or 5V must be present in standby before the computer is started. I tapped 5V from the battery charging LED.
Hope it helps someone.