Fetch Customer data in custom UI extension

authenticatedAccount only returns the customer id, so that makes sense.

Depending on your target, I think you need the use Customer Account API reference.

Example

const exampleCustomerQuery = {
  query: `query {
    customer {
      id
      firstName
      metafields(identifiers: [{namespace: "custom", key: "some_key"}]) {
        type
        value
        key
      }
    }
  }`
};

fetch("shopify://customer-account/api/2025-01/graphql.json", {
  method: 'POST',
  headers: { 'Content-Type': 'application/json' },
  body: JSON.stringify(exampleCustomerQuery),
}).then(response => response.json())
  .then(data => {       
    console.log({data});
  })
  .catch(error => console.error('Error fetching customer data:', error));