Steffen, your configuration in Zabbix frontend is correct.
However, there is a mistake in JSON syntax that you use in the command line. Double quotes after {#THNAME} are missing:
{"data":[{"{#THNAME}:"test_thread"}]}
You should see the error message about it in discovery list:
It should work after that issue is fixed. If not, please provide details about your "@Thread for discovery" macro and the error message that you get.