Non-monetary commissions affiliate

Hi ! I would like to know if we can give one free month of Ghost for our paid members if they become ambassador ?

Example :

  • i have a paid subscriber
  • he is sharing his link to someone else
  • the second person become paid subscriber
  • i want to give one free month to the referrer

I tried with firstpromoter but they said “non-monetary commissions is for custom SaaS , you can’t integrate that with ghost”