I had never tried to implement the HTTP integration, but I tried yesterday to see how it works. As far as I know, you’re right, the Process ID is user-defined and the Authorization is optional. However, I’m not sure about the field URL.
I tried what you were trying to do, and I also got the “502 Bad Gateway” error. Then, I tried to use the URL of a Channel I created some months ago in ThingSpeak (https://thingspeak.com/), and it sends data from TTN to Thingspeak via HTTP (however, you were interested in downlink). Finally I tried to introduce the IP address of a server I created (
http://xxx.xxx.xxx.xxx:8080/) in the URL field and it works, I receive a JSON from TTN and I suppose that sending downlink data to end-devices would not be a problem.
I’m also starting with TTN and so on, I hope these options can help you. Anyway, I’m sure some other people know the answer