Here is a bit of info I’ve collected from the forum for a laser in my future (so not practical knowledge). I’m assuming given the title of this post that you have a Rambo board and are planning control your laser using g-code fan commands. If your laser control logic is 12v compliant, you don’t have to make any Marlin changes. You just pick one of the three fan pins and use M106 and M107 to control the laser. You can find the connections labeled on in this photo: https://www.v1engineering.com/wp-content/uploads/2018/02/Ramboboard.png
If you need 5V logic, then you can reassign one of the fan pins to a 5V PWM pin. You will be modifying this section in pins_Rambo.h:
#ifndef FAN_PIN
#define FAN_PIN 8
#endif
#define FAN1_PIN 6
#define FAN2_PIN 2
The typical advice on this forum is to reassign FAN_PIN by changing ‘8’ to ‘45’, but you should be able to use any of the three fan pins, and you can reassign them to any of pins 44, 45 or 46. Pins 44, 45, and 46 are all PWM pins. Here is a photo of the pins you would then use for the control logic on your laser: Rambo laser pin.
Note if your laser is controlled using a 3.3v signal, you will have to take steps to bring the voltage down using a voltage divider or some other circuitry between the 5v pin and your laser.