There was supposed to be a change released on ther 5th July that passes the Logged in Customer ID to the Proxy App Requests
https://shopify.dev/changelog/app-proxy-requests-include-new-parameter-for-the-logged-in-customer-id
I’m not seeing that on my apps yet, i’d really like to use it as the previous way was extremely clunky.
Is it supposed to be passed as a header, as a get variable or what?