There are many platforms from ManyChat to GetPinPoint that allow you to automate Instagram messages based on keywords in your DMs and/or comments.
Mike. and Kilmer use this feature to automate inviting fans to OnlySteves.
Fans can comment a keyword and automatically recieve a link with a free trial code automatically applied.
This turns the feeling from being sold a subscription, to feeling like they "won" something cool like free access to a paid community!