var request = new GetHoldingsOverTimeRequest()
{
    //Fields = new GetHistoricPositionFields(),
    Fields = new GetHoldingsOverTimeResponseFields(),
    // Args = new GetHistoricPositionArgs()
    Args = new GetHoldingsOverTimeArgs()
};

request.Credentials = CorrectCredentials();
request.identify = CorrectIdentify();
request.Fields = new GetHoldingsOverTimeResponseFields()
{
    Date = true,
    Value = true
};
request.Args = new GetHoldingsOverTimeArgs()
{
    Start = new DateTime(2017,12,31),
    End = new DateTime(2018, 8, 28),
    DisplayCurrencyCode = "SEK",
    PercentageReturn = true,
    Accounts = new Guid[] {Guid.Parse("<BFS Id for a specific account>"), }
};

var response = client.GetHoldingsOverTime(request);
foreach (var responseRow in response.Result)
{
    Console.WriteLine($"Date: {responseRow.Date:yyyy-MM-dd}\tValue: {responseRow.Value:P2}");
}