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

System.Configuration.SettingsProviderAttribute

public sealed class SettingsProviderAttribute : Attribute
Specifies the settings provider used to provide storage for the current application settings class or property. This class cannot be inherited.
namespace System.Configuration { public sealed class SettingsProviderAttribute : Attribute { public string ProviderTypeName { get; } public SettingsProviderAttribute(string providerTypeName); public SettingsProviderAttribute(Type providerType); } }