Wiring heatbed with external controller

Hi everyone!
Since it seems that I can't have my ramps board fully working, I've decided to order an external heatbed controller to handle the high currents for the heatbed (don't know why, but after replacing every component from the mosfet to the polyfuse my heatbed simply refuses to turn on...). On the Amazon page I see that I need to connect the input (which is isolated from the load) to my board, and then either provide the heatbed output of the ramps or a 5-12V signal to switch.
Since my problem is that the heatbed won't have any voltage applied to it, I really can't use the heatbed output from my RAMPS to control it. So I was thinking of using a spare pin + ground to give it a signal.
I have a couple of questions:
- Am I correct in thinking that I can use any unused pin from the arduino? If so, do I get PWM output from any of those or do I have to pick a specific one?
- Once I have my chosen pin, do I just update the board configuration in the firmware so that it knows to give the output to another pin?
Thank you all in advance!

Do you have the right board selected in the Marlin firmware?
There are some that use the D8 heatbed pins for a fan.

#define BOARD_RAMPS_13_EFB 33 // RAMPS 1.3 / 1.4 (Power outputs: Extruder, Fan, Bed)
#define BOARD_RAMPS_13_EEB 34 // RAMPS 1.3 / 1.4 (Power outputs: Extruder0, Extruder1, Bed)
#define BOARD_RAMPS_13_EFF 35 // RAMPS 1.3 / 1.4 (Power outputs: Extruder, Fan, Fan)
#define BOARD_RAMPS_13_EEF 36 // RAMPS 1.3 / 1.4 (Power outputs: Extruder0, Extruder1, Fan)

An external controller usually gets the input switching voltage from the D8 pins and then turns on the high current 12V side of the switch to power the bed, thus taking the 9amp load off D8

Steve