using System;

using System.Text.Json;

public class Program

{

    public static JsonSerializerOptions jso = new JsonSerializerOptions(){

        PropertyNamingPolicy = JsonNamingPolicy.CamelCase,

        WriteIndented = true

    };

    public static void Main()

    {

        /*

        string number = personnummer.Text;          

  var body = @"{

            ""requestId"": ""requestId"",

            ""subjectId"": " + number + " 

            }";

        */

        string number = "ID FROM TEXTBOX";

        var req = new SubjectRequest("requestId", number);

        var body = JsonSerializer.Serialize(req, jso);

        Console.WriteLine(body);

        Console.WriteLine("\nBut if you insist:");

        Console.WriteLine($$"""

                          {

                            "requestId": "requestId",

                            "subjectId": {{number}}

                          }

                          """);

    }

}

​

public sealed record SubjectRequest( string RequestId, string SubjectId );