referrer.js

// check if referrer is outside of the website scope
if (document.referrer.indexOf(`${window.location.protocol}//${window.location.host}`) === -1) {
  window.localStorage.setItem('__fnp_referrer', document.referrer);
}

const referrerField = document.querySelector('[name="signupform[referrer]"]');
const referrerValue = window.localStorage.getItem('__fnp_referrer');

// write stored referrer value back to hidden field in signup-form
if (referrerField && referrerValue) {
  referrerField.setAttribute('value', referrerValue);
}