Creating a self-signed certificate using OpenSSL

$ openssl req -newkey rsa:2048 -sha256 -nodes -keyout private.key -x509 -days 3650 -out cert.pem

The openssl utility will ask you a few details. Make sure you enter the correct FQDN! If your server has a domain, enter the full domain name here (eg. sub.example.com).

ref:https://github.com/python-telegram-bot/python-telegram-bot/wiki/Webhooks