AppSettings.cs

public class AppSettings
{
    public string Path { get; }

    private readonly XElement[] settings;

    public AppSettings(string path)
    {
        Path = path;
        var document = XElement.Load(path);
        settings = document.Descendants("appSettings").Descendants("add").ToArray();
    }

    public string this[string index]
    {
        get
        {
            return settings.FirstOrDefault(e => e.Attribute("key").Value == index)?.Attribute("value")?.Value;
        }
    }
}

//usage
[Cmdlet("Invoke", "Sample")]
public class SettingSampleCmdlet : PSCmdlet
{
    [Parameter(Position = 1)]
    [ValidateNotNullOrEmpty]
    public string SettingPath { get; set; }

    protected override void ProcessRecord()
    {
        var setting = new AppSettings(SettingPath);
        var apiKey = setting["apikey"];
    }
}