Power On Pads
A power button press can be simulated by shorting these two pads. A conductive pair of tweezers or a screwdriver tip will work.
MacBook Pro Unibody https://beetstech.com/blog/where-are-macbook-pro-unibody-power-on-pads-located
It's hard to get access to SMC_ONOFF_L on these machines. To test keyboard functionality, these test points are useful.
Power button test point on the keyboard backside, under the black film (No continuity to SMC_ONOFF_L)
And the power pads are located here on the board: