You can check I AM POP's pricing structure here. You will only be charged when your subscriber count grows beyond 100 subscribers. The first pricing tier is $29 per month, and allows you to reach up to 5000 subscribers. The growth of your subscriber count is completely up to your own promotional efforts, of course.
You can promote your Messenger channel with your unique 'm.me' link. You can share this link on any platform or in any channel, in order to drive traffic to your Messenger channel and gain new subscribers.