How does the new update pass Logged in Customer ID to Proxy App Requests?

[bretto36]

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?