Telegram bot: How to mention user by its id (not its username)

I am creating a telegram bot and using sendMessage method to send the messages.
it is easy to mention user using @username, But how to mention user when they don't have username?

If using the telegram app/web, we can mentioned the user by @integer_id (name), and telegram app/web will convert it into clickable text. integer_id will be generated automatically when we select the user, after typing @.

another background:
I am trying to use forceReply and I want to target user, if they have username, I can easily target them, by mentioning them on the text on sendMessage method.

the bot I am creating is a "quiz" like bot. where each player need to take turn, and the bot is sending them the question, each msg from bot will target different player.

NOTE: I am not disabling the Privacy Mode, I don't want telegram bombing my server with msg I don't need. it was overloading my cheap nasty server. so, disabling it not an option.

I am open for other solution, where the bot can listen to selected player.

thanks.

UPDATE 21/10:
I've spoke to BotSupport for telegram, they said, for now Bots can't mention user without username.

so in my case, I still keep using forceReply, and also, gave a short msg to user which doesn't have username to set it up, so they can get the benefit from forceReply function.