Add items to json

How can I add an item to this json? this way I can receive it in the details activity to complete a url to access it.

[
    {           
        "item1": "Finalizado",
        "item2": "F"
}]

And my intention is to add "item3": "$ data" after "item2": "F" The $ data can be 1, 2, 3, which is a string which will be the value returned by the variable data and this is what I do:

val jorn = items.add(dato) // dato en rojo

val adapter = AdapJor(activity !!, jorn ) //jorn en rojo

private fun requestJsonObject() {

    val queue = newRequestQueue(activity)

    val url = Directions.JORNADAS + Directions.CMP + "&jor=$dato&tmp=" + Directions.TMP

    val stringRequest = StringRequest(Request.Method.GET, url, Response.Listener { response ->

        val builder = GsonBuilder()
        val mGson = builder.create()

        var items: List<ModelJor>

        items = Arrays.asList(*mGson.fromJson(response, Array<ModelJor>::class.java))

        val jorn = items.add(dato)

        recyclerView !!.layoutManager = GridLayoutManager(activity!!, 1)

        val adapter = AdapJor(activity !!, jorn )
        recyclerView !!.adapter = adapter

    }, Response.ErrorListener { error -> Log.d(TAG, "Error " + error.message) })
    queue.add(stringRequest)
}