Where to write a system prompt, when

@app.post("/query/")
def process_text(request_body: dict, db: Session = Depends(get_db)):
    query = request_body.get('query')
    vector_store = QdrantVectorStore(client=client, collection_name="data")
    index = VectorStoreIndex.from_vector_store(vector_store=vector_store)
    query_engine = index.as_query_engine()
    response = query_engine.query(query)
    return response.response