Telegram bots have a deep linking mechanism, that allows for passing additional parameters to the bot on startup. It could be a command that launches the bot — or an auth token to connect the user's Telegram account to their account on some external service.
Each bot has a link that opens a conversation with it in Telegram — https://telegram.me/<bot username>
You can add the parameters start or startgroup to this link, with values up to 64 characters long. For example:
https://telegram.me/triviabot?startgroup=test
A-Z, a-z, 0-9, _ and - are allowed.
your bot will receive a message from that user in this format:
/start PAYLOAD