@georg ,
There's not much info in your description for me to give you a precise answer.
But, given you mentioned "container", I'm assuming that the other system is running on a Docker container in the same host as NiFi is running.
If that's the case, you can use "docker inspect" to find out what the exposed port of the container is and then your URL will be something like either "http://localhost:<port>" or "http://host.example.com:<port>". If TLS is being used, replace "http" with "https".
I hope this helps. If not, please provide more details information about the components you're talking about.
Cheers,
André
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