SelectionFactory set Initial/Default value and display it's placeholder in select/dropdown box

Hi all,

I found this topic here - https://world.episerver.com/forum/developer-forum/-Episerver-75-CMS/Thread-Container/2014/9/Setting-a-ISelectItem-as-selected/ - but none of the solutions solves my problem.

I have this property on a page, which uses custom Selection Factory.

And I want to set initial value and display it's placeholder (in admin view, under "All properties" view while creating new page) here in a select box. I do it like this:

// CustomSelectionFactory.cs:
public IEnumerable<ISelectItem> GetSelections(ExtendedMetadata metadata)
{
    List<SelectItem> result = new List<SelectItem>()
    {
        new SelectItem() { Text = "All", Value = null }
    };
    List<CustomSelectItem> customSelectItems = new List<CustomSelectItem>();
    switch (metadata.PropertyName)
    {
        case "DocumentCountry":
            customSelectItems = Api.GetCustomSelectItems(1).Result; 
            break;
        case "DocumentLanguage":
            customSelectItems = Api.GetCustomSelectItems(2).Result; 
            result.RemoveAll(x => x.Text == "All");
            metadata.InitialValue = "1";
            break;
    }
    foreach (CustomSelectItems customSelectItem in customSelectItems)
    {
        result.Add(new SelectItem()
        {
            Text = customSelectItem .Name,
            Value = customSelectItem .Id.ToString()
        });
    }
    return result;
}

// CustomPage.cs
[Display(Name = "Document Country")]
[SelectOne(SelectionFactoryType = typeof(CustomSelectionFactory))]
public virtual string DocumentCountry { get; set; }
[Display(Name = "Document Language")]
[SelectOne(SelectionFactoryType = typeof(CustomSelectionFactory))]
public virtual string DocumentLanguage { get; set; }

Help appreciated!