<PackageReference Include="System.Configuration.ConfigurationManager" Version="7.0.0-preview.2.22152.2" />

DefaultSettingValueAttribute

public sealed class DefaultSettingValueAttribute : Attribute
Indicates to the provider what default value to use for this setting when no stored value is found. The value should be encoded into a string and is interpreted based on the SerializeAs value for this setting. For example, if SerializeAs is Xml, the default value will be "stringified" Xml.
namespace System.Configuration { [AttributeUsage(AttributeTargets.Property)] public sealed class DefaultSettingValueAttribute : Attribute { private readonly string _value; public string Value => _value; public DefaultSettingValueAttribute(string value) { _value = value; } } }