<PackageReference Include="System.Configuration.ConfigurationManager" Version="11.0.0-preview.3.26207.106" />

System.Configuration.DefaultSettingValueAttribute

public sealed class DefaultSettingValueAttribute : Attribute
Specifies the default value for an application settings property.
namespace System.Configuration { public sealed class DefaultSettingValueAttribute : Attribute { public string Value { get; } public DefaultSettingValueAttribute(string value); } }