PhotoMos relay -calculating the right resistance value and input voltage
You are driving an LED and the rated forward current required is 50 mA maximum or 3 mA to guarantee operation at 25 C. Forward voltage is 1.14 V at 5 mA so I'd operate at 5 mA and you'll need to add a dropper resistor to prevent overload.

Typically from a 5V drive you'll need to "lose" 3.86 V across the resistor at 5 mA. This means the resistor value is 772 ohms so make it 750.