How to check if a user is in a Telegram chat?

I have several chats and one bot in all of them.

I need to send a message about a particular user in a chat, but first I need to check if the user is in fact in this chat, by username. How can I do it?

Hello @Dmitry_Kuchev nice to meet you.

You need to use a Search module to see if that user exists and if the user exist to send the message. Since there is no Search module in the Telegram application, you should read Telegram API documentation, and you can use Make an API call module and search for that specific user to whom you can send the message.