Thanks,
you have put me on the right way.
The documentation is really confusing, but finally, with copy-past and a lot of testing I found the right topic and payload.
Topic: zwembad/rpc ==> Where zwembad is the configured device name, don't forget the /rpc
Payload: {"id":1, "src":"PFC100", "method": "Switch.Set", "params":{"id":0,"on":true} } ==> Where the first ID and SRC is something you choose, this comes back in the confirmation message.
This can be closed