How to pass Json array to WWWform as field in unity -

i want pass 3 field in wwwform, contain 2 single value , json array.

here json:

{      "fname":"abc",      "lname":"xyz",      "marks": [{"marksa":"23","marksb":"65" },      {"marksa":"24","marksb":"56" } ]    } 

my current code is

 void start()     {         wwwform form = new wwwform();         form.addfield("fname", "abc");         form.addfield("lname", "xyz");         //passing array string         string mymarks = "[{\"marksa\":\"23\",\"marksb\":\"65\" },{\"marksa\":\"24\",\"marksb\":\"56\" } ]";         form.addfield("marks", mymarks);         www www = new www("urltoservice", form);         startcoroutine("postrequest", www);     }     ienumerator postrequest(www www)     {         yield return www;         if (www.error == null)         {             debug.log("session saved");         }         else         {             debug.log("www error: " + www.error);          }     } 

but getting 500 internal server error. please me.