how to pass an optional parameter to sql function from C#

function [dbo].[fnKudishikaAmt](@ParishName nvarchar(100),@Hno int,@dateto datetime=Null)
Returns Decimal(15,2)

Select dbo.fnKudishikaAmt('St.George Malankara Catholic Church',29,default)

public double kudishikatotal(string ParishName, Int32 HouseNo)
    {
        String SQLText = "select ChurchDB.dbo.fnKudishikaAmt(@ParishName,@Hno,@dateto) as fnresult";
        SqlCommand cmd = new SqlCommand(SQLText);
        cmd.CommandType = CommandType.Text;
        cmd.Parameters.AddWithValue("@ParishName", ParishName);
        cmd.Parameters.AddWithValue("@Hno", HouseNo);
        cmd.Parameters.AddWithValue("@dateto", "default");
        string rval = GetSingleValue(cmd);
        double kudiamt = 0;
        if (rval != null)
        {
            kudiamt = Convert.ToDouble(rval);
        }
        return kudiamt;
        
    }

private static string GetSingleValue(SqlCommand cmd)
    {
        string ConString = connectionstring();
        string returnvalue = "";
        using (SqlConnection con = new SqlConnection(ConString))
        {
            cmd.Connection = con;
            con.Open();
            returnvalue = cmd.ExecuteScalar().ToString();
            con.Close();

        }
        return returnvalue;
    }