How to call modbus.write_register service from within AppDaemon?

Please can you have a look at my question, still being a relative newbie on HA/AppDaemon…
I wrote a couple of Apps that are working fine with HA. So the basics is working.
I was now writing an App that I want to have call a service to write a register in modbus slave.
My naïve approach to this was to do this:

self.call_service("modbus/write_register", ...

However, this ends with an error:

2021-11-18 16:27:56.854412 WARNING AppDaemon: Unknown domain (default/modbus) in call_service ...

I also tried with the ‘regular’ modbus.write_register that I am using in HA but also doesn’t work.
Am I doing the right think and thinking here? Am I missing something here? Can it be done? How?
Are we looking at 2 totally different kind of ‘services’ both being called ‘services’ here?

Thanks, Rene.

I guess I will use a script for it… if you have a better solution for it please let me know!

Hello I have the same problem. Anybody found a solution?