Add structure (extensionObject) to node-opcua server

Question

I download node-opcua sample server, and I try to add some opc ua nodes, in order to test the server. I was able to create object and variable:

    const namespace = addressSpace.getOwnNamespace();

    const myDevices = namespace.addFolder(rootFolder.objects, { browseName: "TestFolder" });

    const testObj = namespace.addObject({
      organizedBy: myDevices,
      browseName: "TestObj",
      nodeId: "s=TestObj"
    });

    const testVariable = namespace.addVariable({
      componentOf: testObj,
      browseName: "TestVar",
      nodeId: "ns=1;s=TestVar",
      dataType: "Double",
      value: new Variant({ dataType: DataType.Double, value: 1000.0 })
    });

but I was not able to create Struct (ExtensionObject). I tried something like this:

    const testParams = {
      firstField: 0,
      secondField : 0
    };
    const structure = addressSpace.findDataType("Structure");
    const dataType = namespace.createDataType({
      browseName: "TestStuct",
      isAbstract: false,
      subtypeOf: structure.nodeId
    });
    const inner_options = {
        componentOf: testObj,
        browseName: "TestStuctVar",
        dataType: dataType.nodeId,
        value: testParams
    };
    const strucVar = namespace.addVariable(inner_options);

but connecting with UA Expert, I got "Null" dataType: so it seems the server did not add the dataType I created.
With another server, when I add a struct datatype, UA Expert understand the dataType correctly: