Thanks everyone,
Below is the successful input format on the make.tv side:
RTMP-URL: rtmp://${username}:${password}@#.#.#.#/live/
Stream name: live
The "live" app in the URL is what I was missing and what is absolutely required (or any other listening app).
I used "live" in the Stream name, but I don’t believe that it was actually required. I did use testChan for the Stream name once and it showed up in the logs as expected (I didn’t check the GUI, though).