where C.V. is the ON/OFF themostat input of my my central heating system. This allows the Fibaro to switch ON/OFF the heating and works fine. No connection is made between L and IN, so it operates potential free.
I would like to add the possibility to manually switch the relay, so use the S1in the first diagram. However, I see that it is connected to L (mains).
I would definitely check if the switch is rated for 230VAC.
Although the load is extremely low (like really low) if the switch is not rated 230V then it’s possible you get an electric shock.
Like this one HERE you can see it’s rated up to 250V. It looks similar with the one in your picture, thought. Check the spec of yours.