Hello there!
This does not look like the correct api call. It looks like you’re trying to use the assistants api like the chat completions api.
assistants primarily use threads and messages to communicate with the chatbot.
You would likely build an assistant with the api:
https://api.openai.com/v1/assistants/{assistant_id}
save that in code, then create (and save) a new thread that lives inside that assistant with this api:
https://api.openai.com/v1/threads
and then send off a new message to that thread, not the assistant directly.
https://api.openai.com/v1/threads/{thread_id}/messages