I try to pass parameters into aspx.cs page from js script. When I omit:
contentType: "application/json; charset=utf-8"
in ajax request I get by Request.Form["ORDER"] sth like {%7b%22ORDER_ID%22%3a126333%7d}. It means that this data comes to aspx.cs, but it is not decoded.
When I add contentType I get nothing in request. 
Below I attach request.
It is important to read parameters from Request.Form["ORDER"] in aspx.cs;
$.ajax({
        type: 'POST',
        contentType: "application/json; charset=utf-8",
        data: JSON.stringify({ ORDER_ID: orderKeyId }),
        dataType: "json",
        url: sUrl,
        success: function (data) {
            var s = 0;
        },
        error: function () {
            var s = 0;
        }
    });


