"Argument 'domain_name' is required but was not received"

After figuring out that only the params are in JSON and everything else is just a basic parameter list, I am able to invoke services. My problem starts when using the API key: I keep getting this error message: Argument 'domain_name' is required but was not received"

The parameter is there of course, and I've checked the spelling several times. Does this need to be included in the params? Is there any documentation what the module expects when using API keys?