Hello Cor,
As far as I know, you cannot connect directly the Photoelectric Sensor to the digital input of the Shelly 2PM Plus Add-on, because the digital input switches into the minus (gnd). The Photoelectric Sensor you want to use switches into the plus.
- The brown wire is the plus (power supply) 24V dc of the Sensor.
- The blue wire is the minus (power supply) 0V dc of the Sensor.
- The black wire is the switching wire that you could connect to an input on, for example, the Shelly Plus I4DC, the only question is whether the Photoelectric Sensor output does not need a little load to switch properly.
What could possibly be a solution, to have the sensor switch a relay and offer the potential free contact of the relay to the digital input (and gnd) of the add-on, that will work.
Good luck with your project.