I'm not aware of a specific tutorial to do this but you could implement this by creating a Zap in Zapier using their Webhooks feature and calling the webhook from an Execute Code card, passing the user's email. Here's a link with more info:
https://zapier.com/apps/webhook/integrations
BTW, we're working on releasing a direct Zapier integration for Botpress very soon, so stay tuned to our announcements 🙂