You may force nc to wait some time after all data are sent using -q seconds option. As inWhen running "nc" in UDP, nc exits without waiting for reply from the target application (echo_server)
$ echo foobar | nc -q 1 -u 192.0.2.1 4242
to wait for 1 second. Unfortunately the -q option may not be present in your specific implementation of nc. It seems there are a few and their names may vary, e.g. ncat, netcat. I haven't been using Zephyr net-tools project so I'm not sure what's the situation there.
For simple testing it's often enough to type
$ nc -u 192.0.2.1 4242
anything you type into the console will be send to the remote side and the response will be displayed. To exit you'll need to Ctrl+C manually.