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

System.Configuration.SettingsGroupNameAttribute

public sealed class SettingsGroupNameAttribute : Attribute
Specifies a name for application settings property group. This class cannot be inherited.
namespace System.Configuration { public sealed class SettingsGroupNameAttribute : Attribute { public string GroupName { get; } public SettingsGroupNameAttribute(string groupName); } }