<PackageReference Include="System.Configuration.ConfigurationManager" Version="10.0.0-rc.1.25451.107" />

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); } }