How to connect optocoupler module to ABL sensor and Ender 3 board
Connecting is pretty straightforward like the other modules, difference is an extra input lead.

From e.g. here: Module interface description: 


DC+: Positive DC power supply. 
DC- : Negative DC power supply. 
PWM: Signal input (connect MCU port, PLC interface, DC power supply, etc.) 
GND: The negative terminal of the signal 
OUT+: Positive output terminal (connected to the device positive) 
OUT-: Negative output terminal (connected to device negative)


So, 1 and 2 connect to your power supply that matches the voltage of the logic of your microprocessor (5&nbsp;V) as this must be linked to 5 and 6 which are connected to the endstop signal and ground respectively.

As for the sensor, blue is GND, black is signal (PWM) and brown is power as seen in e.g. this answer.