SHELLY 1PM+AddOn with surplus manager

I have solar panels for self-consumption connected to a surplus manager which in turn is connected to a resistance to heat the water in my house as the software I use in the surplus manager, freeds, does not have access by app I intend to connect, in parallel , a shelly 1PM resistance with an AddOn to know the temperature and turn it on if necessary.
How can I make the electrical connections with safety and protections so that when I connect the shelly it doesn't ingest current in the surplus manager?
Thanks