AJAX.NET request handlers - set hidden field

var prm = Sys.WebForms.PageRequestManager.getInstance();
prm.add_initializeRequest(MyPage_initializeRequestHandler);

function MyPage_initializeRequestHandler(sender, args)
{
	var hiddenField1= $get('hiddenField1');

	if (hiddenField1 != null)
	{
		hiddenField1.value = 'test';
	}
}