How do I insert a new key:value set under one of the objects inside AdTagNames in MongoDB?

query.json

{
    "_id" : ObjectId("541945c1a5b1e41035b49ed7"),
    "AdTagNames" : [ 
        {
            "adURL" : "http://www.sdfadgs.com",
            "adPASSBACK" : "http://google.com/passback.html",
            "adTAGS" : [ 
                "sdfgsdfgsdfg"
            ]
        }, 
        {
            "adURL" : "http://www.sdfg.com",
            "adTAGS" : [ 
                "sdfsfsadgfsd"
            ]
        }
    ]
}

User.update({
  _id: xxx,
}, {
  ???
});