You have already specified a namespace in your NodeId; it is the "ns=6" part of it. Are you saying that it does not work somehow? If so, what are the symptoms of the problem?
Note that this is a namespace specified by an index, which is not recommended because the server is entitled to change the indexes between sessions; ideally, you should specify the namespace by its URI ("nsu=..."). For discussion of these topics, see e.g. our Concepts document (comes with the product installation, or:
) - refer to the chapter titled "Identifying information in OPC-UA", subchapter "Node IDs"