JSON.stringify() is important. Sending a JSON object will result in automatic conversion to form/url-encoded key-value pairs.

requesting.js

requests: {
    var json_data = {
        "ticket": {
            "subject": "Hello",
            "comment": {
                "body": "Some question"
            },
            "ticket_form_id": this.carrierOpsFormId,
            "custom_fields": [{
                "id": 23810968,
                "value": "custom value"
            }]
        }
    };
    addTicket: function(json_data) {

        return {
            url: '/api/v2/tickets',
            type: 'POST',
            dataType: 'json',
            proxy_v2: true,
            contentType: 'application/json',
            data: JSON.stringify(json_data)
        };
    },