Referral program integration with Substack?

Hi there,

What are the best ways to run a referral program (to find new subscribers) with Substack? As they don't offer an API it does not seem so easy.

Morning Brew generates well over 1,000 subscribers a day from its referral program (https://medium.com/the-mission/how-morning-brews-referral-program-built-an-audience-of-1-5-million-subscribers-3315482c1aa5), which accounts for 35% of net new sign-ups. The Skimm gets 20% of its subscribers via referral. The Hustle’s referral program drove more than 70,000 subscribers by April 2018 (and is likely well over 100k now).

What kind of tool can I use to run a referral program with Substack? Let me know!

Thank you,