There is nothing built into Discourse that will automatically post a reply to a topic, but you could accomplish this either with a plugin, or with the Discourse API.
For the API approach, you could create a Discourse webhook that is triggered by a Topic Event with your introduction category set as the triggered category. You would then need to set up an application to listen for the webhook and create the reply. Something as simple as a WordPress site could be used for this.
You could try using Zapier to listen for the webhook and create the reply. There is a general overview of how to do that here: How to make requests to the Discourse API with Zapier. To create a Zap that auto replies to topics, you would use a Zapier webhook both for the trigger and the action step of the Zap. There are details about setting up a Zap’s trigger step to listen for a Discourse webhook here: Discourse Webhooks and Zapier.