Hey @Elisa Capitanio7! Unfortunately, we don't currently have a built-in way to track referrals of email list subscribers (but, feel free to file that as a feature request here7 — we always appreciate the feedback!).
At this time, you'd need a 3rd party integration to achieve this. We have a list of our official integration partners here10, but if you find a service that offers this function, you may also be able to connect it to ConvertKit via Zapier9 even if they don't have an integration with us yet :)