Hi Steven

You can add the categories property to your indexed object(s) as follows:

[ModuleDependency(typeof(EPiServer.Web.InitializationModule))]
public class AddGetaCategoriesInitialization : IInitializableModule
{
    private static readonly ILogger Logger = LogManager.GetLogger();

    private bool _initialized;

    public void Initialize(InitializationEngine context)
    {
        if (_initialized || context.HostType != HostType.WebApplication)
        {
            return;
        }

        SearchClient.Instance.Conventions.ForInstancesOf<PageTypeWithGetaCategories>().IncludeField(x => x.GetaCategories());
        
        _initialized = true;
    }

    public void Uninitialize(InitializationEngine context) { }
}

Then use the extension method as following:

public static List<string> GetaCategories(this object content)
{
    if (content is PageTypeWithGetaCategories page)
    {
        return ConvertToList(page.GetaCategoriesPropertyName);
    }
    return new List<string>();
}

Then facet as usual using .TermsFacetFor(x => x.GetaCategories()) and query using  query = query.FilterHits(x => x.GetaCategories().Match(categorySearchTerm);

David