postconfigure4.cs

IConfigurationSection mySettingsSection = builder.Configuration.GetSection("MySettings");
builder.Services.Configure<MySettings>(mySettingsSection);

services.PostConfigure<MySettings>(options => 
{
  if(!options.MyOptions.Any(x => x.Alias == "itemOne"))
  {
    options.MyOptions.Add(new MyOption { Alias = "itemOne", Value = 0 });
  }
  
  if(!options.MyOptions.Any(x => x.Alias == "itemTwo"))
  {
    options.MyOptions.Add(new MyOption { Alias = "itemTwo", Value = 0 });
  }
});