using System;

​

public partial class MyNotification

{

    public string MyField { get; set; }

}

​

public partial class MyNotification

{

    public record Metadata(int PartnerId);

    public Metadata _Metadata { get; set; }

}

​

public class Program

{

    public static void Main()

    {

        var obj = new MyNotification

        {

            _Metadata = new MyNotification.Metadata(5),

            MyField = "foo"

        };

        var opt = new System.Text.Json.JsonSerializerOptions()

        {

            WriteIndented = true

        };

        Console.WriteLine(System.Text.Json.JsonSerializer.Serialize(obj, opt));

    }

}