Just checking as I notice that the documentation link you posted is for the Cloud version but you say you're on the Server version. The APIs might not be interchangeable between the versions.
If that's working for you, and the JSON file you're providing is in the correct format as mentioned in the documentation here (smartbeartm4j / tm4j-junit-integration — Bitbucket) then I'm sorry I can't help. I've only used the APIs for GET functions and not very often.
thanks for the answer! yes you're right, I checked the cloud documentation because it's written how to make the integration with pytest, and in the server documentation it isn't reported. Is it impossible? but I used the server version API that I found here https://support.smartbear.com/zephyr-scale-server/api-docs/v1/
Glad that's helped clear up part of the issue. I've not used the APIs that much, and not for what you're trying to do but it seems to me that the API for server is generic and as long as you follow the syntax it should work. Has it worked for you?