How to trigger an Function App on Azure Relay

I have successfully connected an Asp.Net Core app to the Azure Relay over a hybrid connection but now I need to run custom logic on each http request made to the relay. My idea is that all the https requests made to the Relay get passed over to the Relay Host running on the premisses. This Host in turn passes the https request to the Asp.Net Core application. Works like a charm!

Now I want to run custom logic on Relay. I want to check the request header and based on this check I will either pass the request onward or immediately return a response saying something went wrong.

How can I do this. I found numerable examples of Function App on HybridConnection endpoint but the problem is that my current Hybrid connection defined in the Relay is not available for choosing.

Is there any way to do this?