getting the value entered in the radprompt in the serverside

Hi,


can anyone please tell me how to get the value entered in a radprompt on the serverside? I have the below javascript:


function openprompt()
{
("please enter the file name",CallbackFn)
}
function CallBackFn(arg)
{
alert(the element entered is"+arg)
}


I have to get the value "arg" to server side.how to do that...