Once you have created your FirstPromoter account, go to the Settings menu and to the Integrations tab.

Copy your Account Id and paste it into Ghost Admin on the Integrations page for FirstPromoter:

Hit Save and you're done! You can now start inviting referral partners to join your program so they can start recommending your publication to gain rewards!

Further reading

Official Ghost + FirstPromoter Integration
Launch a referral program for your membership business with Ghost and FirstPromoter & offer rewards, competitions and payouts to your most loyal Members!