HTTP Referrer on Page Load

I am interested to capture and log (on the server side) the referrer to my Dash app.
Naturally, checking the request object inside the callback is too late; the referrer is the page itself by then.

I notice that it is possible to override the client-side dash-renderer and introduce hooks.

Is there a mechanism which can be used on the server side which could be used to introduce a hook when the initial layout is requested.

Thanks in advance for any hints/tips!