Setting value of [SelectMany] property programmatically

I've a string property on my Page with the [SelectMany] attribute, and I need to create the page and fill the property programmatically. 

    [CultureSpecific]
    [SelectMany(SelectionFactoryType = typeof(MyItemsSelectionFactory))]
    public virtual string MyItems { get; set; }

at the moment I'm using the following for setting the value of [SelectOne] properties...

    myObj.MyItem = "99" // where 99 == ContentReference.ID of target page

which works ok, but how do i set multiple values at once?

EPiServer.Shell.ObjectEditing.SelectManyAttribute