Jquery – Get Classic ASP variable from posted JSON

I'm trying to post JSON via AJAX to a Classic ASP page, which retrieves the value, checks a database and returns JSON to the original page.

I can post JSON via AJAX. I can return JSON from ASP. I can't retrieve the posted JSON into an ASP variable.

POST you use Request.Form, GET you use Request.Querystring. What do I use for JSON?

I have JSON libraries but they only show creating a string in the ASP script and then parsing that. I need to parse JSON from when being passed an external variable.

Javascript

var thing = $(this).val();

$.ajax({
         type: "POST",
         url: '/ajax/check_username.asp',
         data: "{'userName':'" + thing + "'}",
         contentType: "application/json; charset=utf-8",
         dataType: "json",
         cache: false,
         async: false,
         success: function() {
            alert('success');
         }
});

ASP file (check_username.asp)

    Response.ContentType = "application/json"
          sEmail = request.form() -- THE PROBLEM

          Set oRS = Server.CreateObject("ADODB.Recordset")
          SQL = "SELECT SYSUserID FROM dbo.t_SYS_User WHERE Username='"&sEmail&"'" 
          oRS.Open SQL, oConn
          if not oRS.EOF then 
            sStatus = (new JSON).toJSON("username", true, false)
          else
            sStatus = (new JSON).toJSON("username", false, false)
        end if
response.write sStatus