var client = GetClient();
var session = await client.CreateSessionIDAsync();
var sessionID = session.@return.SessionID;
var pl = 73;
var katowice = 28191;
var katowickiPark = 412;
var asciiSearch = false;
var positionPoint = new TSoapTLongLatPoint()
{
    Latitude = 21,
    Longitude = 52
};

var pointPositions = new TSoapTLocalizeObjectPositionData[1]
{
                new TSoapTLocalizeObjectPositionData()
                {
                    PositionPoint = positionPoint
                }
};

var stringFields = new string[1][];
stringFields[0] = new string[1] { "The only string" };

var doubleFields = new double[1][];
doubleFields[0] = new double[1] { 1.1 };

await client.SearchInitializeAsync(sessionID, asciiSearch);
var citiesResponse = await client.SearchSelectCitiesAsync(sessionID, pl, "", "", "", "", "");

var itemsResponse = await client.SearchSelectItemsAsync(sessionID, katowice, 0, "");
var objectsResponse = await client.SearchGetItemsListAsync(sessionID, katowickiPark, 2);

var localizeResponse = await client.LocalizeAddObjectPositionsAsync(
    sessionID, 1, pointPositions, stringFields, doubleFields);
// returns -203 not matter if EntityID = 0 (index in ObjectsResponse) or 421 (index in a static list) still returns -203

Console.WriteLine($"Server response code: {localizeResponse.@return.ToString()}");