Configuring classMap for included subcollection

I have this object model:

PhoneBookContact contact2 = new()
{
    //Id = ObjectId.GenerateNewId().ToString(),
    FirstName = "contact 2",
    Location = "Bern",
    PhoneBookIds= [], //Ids of another collection
    Numbers = [
        new PhoneBookContactNumber { /*Id = ObjectId.GenerateNewId().ToString(),*/ Number = "+41580000003", Type = NumberType.Office },
        new PhoneBookContactNumber { /*Id = ObjectId.GenerateNewId().ToString(),*/ Number = "+41760000004", Type = NumberType.Mobile }
        ]
};

All properties called Id are strings, but should be stored as ObjectId in the database. Do do this for all Id properties in all collections, I’m using a Convention

public void PostProcess(BsonClassMap classMap)
{
    var idMemberMap = classMap.IdMemberMap;
    if (idMemberMap == null || idMemberMap.IdGenerator != null)
        return;
    if (idMemberMap.MemberType == typeof(string))
    {
        idMemberMap.SetIdGenerator(StringObjectIdGenerator.Instance).SetSerializer(new StringSerializer(BsonType.ObjectId));
    }
}

To treat the PhoneBookIds, I’m doing this:

            classMap.MapProperty(x => x.PhoneBookIds)
                .SetSerializer(
                    new EnumerableInterfaceImplementerSerializer<List<string>, string>(
                    new StringSerializer(BsonType.ObjectId)));

Now I’d like to have the same treatment for PhoneBookContactNumber.Id. So I tried this:

classMap.MapProperty(x => x.Numbers[0].Number).SetSerializer(new StringSerializer(BsonType.ObjectId));

which doesn’t work. Any idea what I have to do to autogenerate the Id for that included subdocument?