var chatId = message.Chat.Id;
var list = new List<KeyboardButton>() { new KeyboardButton("Пополнить баланс"), new KeyboardButton("\U0001F3E0Главное меню") };
var markup = new ReplyKeyboardMarkup(list);
await botClient.SendTextMessageAsync(chatId, "Hi!", replyMarkup: markup);
var markup
таким образом:var markup = new ReplyKeyboardMarkup(list, resizeKeyboard=True);