$(".act_btn").on("click", function(){
    var act_id = $(this).attr("data-id");
    startRecordItem = new StartRecordItem({
        activity: act_id,
    });
    startRecordItem.save({
        success: function(response){console.log(response)}
        });

    /*
    recordItem = new RecordItem({
        id: ... <---- I have to populate this with the data from the server response.
    });
   */

{"activity": 1, "id": 14}