If you want a check to be activated only by a specific user, enter the recipient’s @username next to the amount while creating a check via inline mode (e.g. “@send $10 @durov Thank you”).

If the recipient has no username, choose Edit Restrictions in the check settings, then tap Set Recipient and forward any message from the user.

Note: The user must has disabled anonymous forwarding in Telegram's ‘Privacy and Security’ settings.