Just apply where the metal arm makes contact with each other. Press the pedal with your hand, and you can see where on both side of the pedal arm there is some parts that overlap. Spray a bit of silicon spray (or lithium grease like you say, but I use(d) wet PTFE lubricant) in there and press a few times and you should have it fall in between where the squeak is coming from. It was super annoying for me too. At one stage I used dry silicon spray but this only lasted a day or two. Then I bought Wet PTFE lubricant (spray can with long nozzle) which is the same thing but wetter, and it lasted for months. Barely needed to touch it again. I used lithium grease for the brake assembly and the PTFE for the clutch and throttle.
Good luck!