Using the force join option, you can force users to join specific channels and/or groups in order to use the bot.
To add a new channel (or group) you need to firstly add your bot as administrator in the chat.
Then you need to copy the link of one of messages in your chat and send it, as you can see in the video:
Users will need to join the specified channels/groups before seeing the bot start message:
If a user is already subscribed in all the specified chats, he will obviously have direct access to the bot.