How to make nested objects into array in WWWForm

I need to send a json object like this to the server:

{
   categories": [
        {
            "owner": 0
            "category_id": 7074
        },
        {
            "owner": 1,
            "category_id": 18162
        }
    ]
}

But with the built-in AddFiled function, I can’t do it in WWWForm. How to do it?