You can edit the customer account page in your theme templates, it's usually called something like customers/account.liquid.
You can add the code / referral link(s) you want to in that file.
Founder of Speed Boostr (Shopify optimization and dev agency). Creator of Theme Scientist (A/B testing app). Creator of Order Automator (app that auto tags + fulfills orders, send emails). Creator of Tip Jar (add a tip button to your store). Creator of Shopify Analyzer (free performance analysis tool). My team also builds custom apps and automated solutions. Enjoy the adventure!