The pot is an integral part of the 555 timing circuit, it's not something that can be directly replaced by a voltage signal in that unit.
But: You should be able to feed a PWM signal to the pot connector wire nearest the centre of the board, if you leave the pot disconnected and remove the timing capacitor that connects between pins 2/6 to negative.
As long as that is high enough level, it will trigger the 555 thresholds and make that give the same signal out, but inverted.
That end pot pin appear to go to 555 pins 2 & 6 via a resistor. Driving those pins, without a timing cap, the 555 will just work as a schmitt trigger and turn on & off at around 1/3 and 2/3 its supply voltage.