OPC-UA - Method call with "complex" input values

While trying to call the Acknowledge Method function, we ran into problems with passing a ByteString and a LocalizedText as method arguments.

We activated logging for Conversions and call methods as proposed above but got no additional hints on how to supply the arguments in a proper format.

We tried to import ByteString via:

from org.eclipse.milo.opcua.stack.core.types.builtin import ByteString
But milo was not available from within the alarmAcked method.

Do we have to provide the method arguments in JSON format and if so, is there a schema to follow?