Learn information about users in telegram?

Question

API

Welcome all.
Basically rummaged through everything. Didn't find anything.
The point is, there is a telegram username. I want to send him a letter. (username - this user has already communicated with the bot).
I read off api, about sending messages, it is written that you can specify @username in chat_id . But the result is 0.
Writes that chat_id was not found.
How can I find out the user_id of the user using api? (API software, no need to throw bots that do it.)