So you want to retrieve the result of the method toString() of the LogOnModel instance which is passed as first parameter right ?
Did you get something as result (even unexpected value) when testing or is there no data gathered at all ?
By the way, you've specified the full classname of the second and the third parameters as System.string; as far as I remember c# classnames are case sensitive, so let's change it to System.String (I'm not sure it explains your problem but give it a try).
Also, be sure to keep the actual parameters order of the method you want to instrument.
You can also check your bytecodeTransformer logs on the java agent to make sure that the method is properlly instrumented.
Grep on classname/method, you should find something like :
Applying method interceptor data.POJOGatherer at <classname>.<method_name>
Then check if the instrumented class has been properly verified. (if not, the agent will rollback the bytecode injection)
Look for something like "<classname> has been successfully verified"
++
Edouard