$("#btn").click(function() {
     $.ajax({
         type: "GET",
         url: "/api/Product/id",
         beforeSend: function(request) {
            request.setRequestHeader("token","666");
         },
         success: function(result) {
             alert(result);
         }
     });
 });

$.ajax({
    headers: {
        Accept: "application/json; charset=utf-8"
    },
    type: "get",
    success: function (data) {
    }
});