Following a link with the start parameter will open a one-on-one conversation with the bot, showing a START button in the place of the input field. If the startgroup parameter is used, the user is prompted to select a group to add the bot to. As soon as a user confirms the action (presses the START button in their app or selects a group to add the bot to), your bot will receive a message from that user in this format:
/start PAYLOAD