Custom Integration Object as Output in custom Business Service invoked in REST API

Dears,

we need to invoke a custom BS via REST to retrieve records provided by a custom IO.

We have different custom IOs and we need to set as BS' output one of them according to the Inputs properties during the call.

In order to set the needed IO we are using scripting:

var newPS = TheApplication().NewPropertySet();

newPS.SetType("SiebelMessage");

newPS.SetProperty("MessageId","");

newPS.SetProperty("MessageType","Integration Object");

newPS.SetProperty("IntObjectName",< IO name>);

newPS.SetProperty("IntObjectFormat","Siebel Hierarchical");

Outputs.AddChild(newPS);

the problem is that with this piece of code we get only the following response from REST

{

  "SiebelMessage": {

    "IntObjectFormat": "Siebel Hierarchical",