I have the same problem.
The trick I did was to go to the Chat playground and inspect the network requests when sending a message - you can see the endpoint used that way.
It will most likely have this format:
https://<resource>.openai.azure.com/openai/deployments/<deployment name>/chat/completions?api-version=2024-08-01-preview
The API version might be different for you depending on the fine-tuned model, so check the network request to make sure.