使用字符串串联.

String message = "this is value  want to pass to the ActualMessage attribute " ;
String input = "{\r\n" + 
               "\"Level\": 0,\r\n" + 
               "\"Name\": \"String\",\r\n" + 
               "\"msgName\": \"String\",\r\n" + 
               "\"ActualMessage\": \"" + message + "\",\r\n" + 
               "\"TimeStamp\": \"/Date(-62135596800000-0000)/\"\r\n" + 
               "}" ;