ASP.NET - Ajax.BeginForm OnSuccess call back with params

Since you're using get_response() I'm guessing that you're not using the unobtrusive javascript stuff (in MVC3 you've set HtmlHelper.UnobtrusiveJavaScriptEnabled = false) and you're referencing the MicrosoftAjax,js and MicrosoftMvcAjax.js files. If that's the case you just need to drop the new keyword.

 using (Ajax.BeginForm("Register", new AjaxOptions() { OnSuccess = "function(arg){HandleBasicForm(arg , 'MyCustomVariable')}"})

If you are using the MVC3 unobtrusive javascript support with jquery.unobtrusive-ajax.js then you can use the implicitly available xhr and data variables instead.

using (Ajax.BeginForm("Register", new AjaxOptions() { OnSuccess = "HandleBasicForm(data, 'MyCustomVariable')"})

In your handler there would be no need to use get_response().get_object() since the deserialized JSON data would be passed in to your handler directly.

function HandleBasicForm(data, myCustomVariable){    var someValue = data.someProperty; //work with data object returned    ....}

OnSuccess receives data, status, xhr from the server:

OnSuccess = "myJsMethod(data, status, xhr)"

And then its equivalent JavaScript method would be:

 function myJsMethod(data, status, xhr) {}

Now your controller should return:

return Json(new { param1 = 1, param2 = 2, ... }, JsonRequestBehavior.AllowGet);

Then in myJsMethod you will have access to data.param1 and so on.