In this case for example:

using(var c = new NpgsqlConnection("Server=127.0.0.1;Database=XXX;User Id=YYY;Password=ZZZ;AlwaysPrepare=True"))
{
c.Open();

using(NpgsqlCommand cmd = new NpgsqlCommand())
{
cmd.Connection = c;
cmd.CommandType = CommandType.StoredProcedure;
cmd.CommandText = "sp_test";
cmd.Parameters.Add("@ttt1", NpgsqlDbType.Integer).Value = "1";
cmd.Parameters.Add("@ttt2", NpgsqlDbType.Integer).Value = "2";
cmd.ExecuteNonQuery();
}
}